Ain’t is spelled that way by Oxford Concise Dictionary.

Fowler defends “ain’t” or “an’t” as an abbreviation for “am not I”, as in “ain’t I pretty?” We tend to say “aren’t I pretty”, which is even less defensible.

Correction, William: “Aren’t I purdee!”

hhp

why thank’ee y’all fer sharing thet wisdom on right-talkin’

:o)
…
-=®=-

I think what Fowler was probably getting at in William’s comment above, is that ain’t completes a very productive syntactic paradigm in English (contractions of pronominal + be + negation):

You are not; You’re not; You aren’t
He is not; She’s not; It isn’t
We are not; We’re not; We aren’t
They are not; They’re not; They aren’t
I am not; I’m not; *I amn’t / ?I ain’t

Not that most folks use ain’t in precisely that manner—but that’s what Fowler was suggesting over the ridiculously ungrammatical aren’t I.

and everyone agrees on the meaning…

I think that may actually depend on where you hail from. In the southeastern U.S. (where I grew up) people use ain’t to mean both isn’t/amn’t as well as hasn’t/haven’t. If you listen closely, some educated Southerners (they do exist!) still differentiate the two by aspirating the second: hain’t.

I hain’t had enough sleep, so I’m going to bed y’all.

I lay it won’t be long till “ain’t” ain’t substandard no longer.

The prescriptivists of the 19th century forced “ain’t” out of standard speech. My guess is that the relatively obscure relationship between “am not” and the contraction bothered some people (the form had formerly been “an’t”). Then probably in reaction to this, people began to use “ain’t” in other meanings—“isn’t”, “hasn’t”, etc.

More on this: http://en.wikipedia.org/wiki/Ain’t

Now the stigma attached to the word is so great, and its use so variable in meaning in different groups, that it’s hard to see this come back in its original, logical meaning. The prescriptivists ruined a perfectly good word, innit?