I'm pretty sure I've heard elementary school-aged kids use it.

"A hundred" is just another way of saying "one hundred", and presumably "He got a one hundred on the test" is okay, right? I don't think I've ever said "an a hundred", but I can't completely exclude the possibility.

How would you read "He got a 100 on the test"? Would you really read "100" as just "hundred"? Are there any other contexts in which you'd read it that way?

I'd go so far as to say that I think I've used/heard "an ahundred" more than just "a hundred", but only in similiar contexts, as a noun. Just my two cents worth.

Two ahundred rups for a bottle of fake poo is certainly not correct.

... but if it is genuine, it might be worth the money:
Zoo auctions tiger poo
http://www.thisislocallondon.co.uk/display.var.914877.0.zoo_auctions_tiger_poo.php
“The park believes it will scare off cats and foxes if placed in people's gardens.”

How would you read "He got a 100 on the test"? Would you really read "100" as just "hundred"?

Yes, I really would, because the written form is just a representation of the spoken sentence "He got a hundred on the test." If it were simply written "He got 100," it would be ambiguous between "a hundred" and "one hundred"; the a is needed for disambiguation.

Googling "an ahundred" and "an a-hundred" turn up quite a few hits, actually, even allowing for the written dialect pronunciations where "an a hundred" turns out to be "and a hundred" or even "than a hundred". It doesn't seem to be formal anywhere, though. I can't say whether I've ever said it - who pays attention? - but I'm pretty sure I've heard it. I think "He's got a temperature of an a hundred and two" sounds quite familiar. I don't think anyone I know would write it, though. Heck, isn't that what numerals are for?

Only now do I realise that in some languages there is neither “a” nor “one” to mark the unity. For example it's il a eu cent (not “un cent”) or une longueur de mille millimètres, and it's when they are two or more that the hundreds and thousands literally begin to be counted — deux cents dollars, trois mille mots, etc.

Incidentally, what's the difference between “a hundred” and “one hundred”? I've counted several times on my fingers and each time I've had the same result.

I always say "That's a whole nother story, with the "whole" inserted into "another", but I have never dared write it even though I've often wanted to.

I also have seen a Sono-English sign "Danger / Wei Xian" where the English word was treated as a binome, "Dan + Ger".

The Slavic languages don't count a single hundred or thousand either. In German it's regional (I, in Austria, never say "einhundert"). In Chinese -- uh, Mandarin, anyway -- it's obligatory, BTW; this may be related to the fact that for 200 or 2000 the form of "2" that goes with classifiers is used, not the ordinary numeral.

The "a whole nother" phenomenon may not be unique. In my dialect it is usual to say what translates at "a such a".

I always say "That's a whole nother story

Me too, and for a long time I didn't even realize it was "wrong" -- it just sounded so natural! I presume that's the case with the "an ahundred" folks, but that one sounds strange to me.

Fanf***n'tastic. Just another example of re-ordered words becoming other words. Another whole story of a whole other story.

Isn't that tmesis?

"Isn't that tmesis?"

Dang, beat me to it. One of the finest words in the lexicon.

Actually the most bothersome example for me, for some irrational reason, is insertions in the middle of "half an hour"--"I've waited half a bloody hour" is all wrong--what you would really want is "I've waited half a bloody n'our".

Battle Hymn of the Republic, Julia Ward Howe

"I have seen him in the watchfires of an hundred circling camps"

(What will I be capable of when I've had TWO coffees this morning?)

That's just the standard traditional treatment of h- as a vowel; you see it throughout 19th-century works.

To paraphrase an old chestnut: "Sham poo for my real friends and real poo for my sham friends."

If you haven't heard the original, here's a hint: it's a toast.

"The Slavic languages don't count a single hundred or thousand either." In formal Russian, "одна тысяча" can be used, but not so with сто. (Unless it's одна сотня, where сотня is a noun indicating something made up of a hundred units.)

"That's just the standard traditional treatment of h- as a vowel." As mute, perhaps?

Yes, exactly.

Hm, funny. "An ahundred" really doesn't sound unnatural to me, at least in the cases he's used it in. I wonder if I've used it...!

If it doesn't sound funny to you, you almost certainly have, I'd say.

OK, I'm a college professor and I hear "an a hundred" a lot with respect to grades. I think it is an extension of the usage "an A," "a B," "a 93" etc. In this case "a hundred" is not used as a quantity but as a grade. It only sounds funny because of the reduplicated article: "I got a one hundred on the test" doesn't (I think) set off our language-error radar in the same way. Some proof of this may be in the fact that I've never heard the article used when grades are expressed as a percent ("I got 83 percent" not "I got an 83 percent").

Just caught myself saying "an ahundred dollar bill" ;)