I grew up in Balto and graduated high school in 2004, so I've heard and used this meaning of 'yo', but I didn't know it wasn't universal.
The paper in the Liberman post reports middle-school students saying that 'yo' is used for boys and 'shorty' for girls. For me 'shorty' can be used for boys or girls, although it has a slightly disparaging connotation when referring to boys, implying youth and immaturity, and is more commonly used BY young men.
The antecendant of 'yo' is more often male, but doesn't have to be. For example, if a girl crosses over a boy in basketball, you could say, 'yo broke his ankles'. It is obvious and unambiguous in this sentence that 'yo' refers to the girl, since 'to break [someone's] ankles' cannot be reflexive.

Wild! Let's hope this spreads, so we can stuff "he or she", "him and/or her", etc. etc. down the oubliette once and for all.

From the examples, sounds more like reanalysis of yo from an interjection to a noun in vocative case and then a noun in general.

Yo would be easy to confuse with you.

Singular they is already entrenched as the neutral singular pronoun; it just hasn't been accepted in formal style, and any neutral singular problem would face the same problem.

I taught 7th grade English for a year in Kansas City, KS (ending about 19 months ago) and this exact sense of "yo" was used there in exactly the way described, particularly by African-American boys. It took me ages to figure out that it replaced "he" and "she." I don't know that I ever heard a girl use it, and I should also note that it was almost always used in an accusatory sense - a kid would march up to my desk and point at someone in the class and yell, "Yo took my pencil!"

What the fuck.

A new personal pronoun out of fucking nowhere.

This is earth-shattering.

Is it possible this morphed from y'all?

A new personal pronoun wouldn't be earth-shattering in Japanese... but then I'm not convinced it's a pronoun yet.

The first place I ever heard "yo" was as response to a roll call, instead of "here" or "present". Maybe "yo" is a demonstrative meaning "here"; that would fit with the usage to get someone's attention. To fit the newly reported usage, it would have to shift from proximal to distal to mean "(person or thing) over there".

I've also heard "yay" meaning "this much". (Not sure what part of speech this would be called) E.g. "The water was yay (speaker holds palm horizontally in front of body to indicate) high."

It didn't, Martin.

I never mastered Middle English, but my memory is that pronouns were jumping around all over the place, so that you had to know context, dialect, and period in order to be sure what was meant. Likewise Old French, if I'm not mistaken.

caffeind,

I've heard "yay" used that way as well - it seems rural to me and could be a regional New Hampshirism, but it may be some odd dialect word from somewhere else that gets transmitted in jokes. It's just another word for "so".

Not sure what's so earth shattering about a new personal pronoun - especially for third person.

I thought 'yay' as in 'yay high' (obligatorily accompanied by a palm down gesture?) was just general american english (spoken not written).

"I've also heard "yay" meaning "this much". (Not sure what part of speech this would be called) "

I always thought this 'yay' was a form of 'here' >'hyeah' as in '[this] here much'.

Not sure what's so earth shattering about a new personal pronoun - especially for third person.

I don't know if "earth-shattering" is the mot juste, but considering there hasn't been one since the Middle Ages, it's pretty surprising.

I always thought this 'yay' was a form of 'here' >'hyeah' as in '[this] here much'.

OED: "[Prob. f. YEA adv.]" (The first cite is from 1960, which surprised the hell out of me, but I'm sure they've got antedates in hand awaiting the revision of the letter y.)

It was used in the plural in Spike Lee's "25th Hour" by a dope fiend in New York complaining about having to go to Harlem to cop: "I can't go up there. The yo's will eat me alive." Or something along those lines. I've actually heard it used that way myself in not such nice neighborhoods. For some reason the phrase "them little yo's" keeps coming to mind.

Idle, utterly amateur speculation: it may have something to do with the cacophony of attention-focusing-device-yo's one hears in those not such nice neighborhoods. You want to refer to the people making all those yo's...

If I understand you correctly, jamessal, then you're talking about a completely different usage. The 'yo' I think you're talking about is an epithet used (mainly by young suburban whites?) to refer to young black men who they identify with the 'hip-hop' or 'urban' cultures. I would guess that it's related to 'yo-boy'.
The word in question here, on the other hand, is a pronoun meaning 'he' or 'she', and is used by young people in Baltimore City schools. These students are probably 80-90% black, and many come from 'those not such nice neighborhoods' you mentioned (black and white). So I don't think it's being suggested that these kids are borrowing a collective racial term used by another social group for themselves and turning it into a third-person singular pronoun.

I don't think you do understand me, but that's my fault; I didn't realize that "yo's" in the Spike Lee movie probably was used as an epithet (which is missing a lot, I know -- my head was somewhere else). But when I heard it used personally, mainly in Trenton (I don't know why I was being so cutesy about it before), it wasn't an epithet at all. It was just street slang, in this case black people referring to other black people.

I'm sorry. I'm all over the place today. The use I just described is still an epithet, of course, just not a derogatory one. The point I was trying to make earlier (if I had one at all) was that maybe there's a connection between the epithet and the pronoun. It didn't seem like a big stretch to me that somebody says "yo" so somebody else calls him "yo" and then it happens again and again until it becomes kind of generic and then you've got a pronoun. Though maybe I'm just totally off base.

I'm guessing the two uses are related, but it's not clear exactly how. Ve must haff investigations!!

Why hasn't anyone cited The Wire? I don't know how accurate its depiction of Baltimore street slang is, but the police characters in the show are constantly referring to African-Americans generally or young male drug-dealing footsoldiers in particular as "yos." Don't they talk about "yos on the corner" or "that tower yo"/"that project yo" the way they might say "project n****r"? This fits with the yo/shorty distinction but is not an example of the neutral pronoun.

Agree with Hopper, I'm familiar with that usage from "The Wire" as well. "Yo" as a noun appearing to mean approximately "street kid" or "miscreant youth", and occasionally used as an adjective, as in, "This [disposable phone] is the latest in yo technology".

If that usage is remotely accurate, I'd guess this is simply what that Language Log entry calls "an instance of the general vernacular practice of using descriptive adjectives -- especially those that are used as nicknames or vocative epithets -- in a quasi-pronominal way." Which I'm understanding to mean that a noun "yo" would be usable in the same way I might say "Idiot stole my parking place!" instead of "He" or "That idiot".

Does anyone know of any evidence that this is actually a new pronoun, instead of a "vocative epithet" serving a pronominal function?

Incidentally, I really enjoy The Wire for its writers' great ear for dialect, among other great elements. Worth checking out!

I hate to keep stressing this, but y'all are talking about a different usage from a different sociolinguistic group. I don't have the documentary evidence or the linguistic training to declare that the 'yo' in question is a new pronoun, but I know it's not the same word used by cops and such to refer to 'miscreant youths'. The only connection is that they both probably derive from 'yo' meaning 'you'.
I guess saying "take my word for it" isn't going to work?

Maybe not?

I picked up (somewhere along the way, most likely Brooklyn) saying 'yo' at the end of sentence. "Whatever yo"--with 'yo' being basically equivalent to 'man' or 'dude'.

So, a usage quite similar to what Emily pointed out above. I'm feeling the vocative epithet serving a pronominal function-morph (yo).

David Simon's 1988 book Homicide features "yo" a number of times. I got the impression from the book that the term was an epithet the cops had for African-Americans.

I don't use it often, and I don't know where and when I picked it up, but I've used "about yay high." It can't be (usefully) used in formal writing because it must be accompanied by a physical gesture showing how high. I could see it used in dialog in fiction, but the author would still need to refer to a physical gesture.

A new personal pronoun wouldn't be earth-shattering in Japanese...

Of course, but Japanese cheats...

Maybe "yo" is a demonstrative meaning "here"; that would fit with the usage to get someone's attention. To fit the newly reported usage, it would have to shift from proximal to distal to mean "(person or thing) over there".

Oh. So a third-person pronoun arising out of a demonstrative maybe-pronoun... that's not exactly out of fucking nowhere.

"Idiot stole my parking place!"

Ah. I see. That makes sense. I wasn't thinking beyond Standard Average European.