Actually, ain't IS a word, just not in modern use. It was used by the upper class as a substitute for 'am not'. I'll have to get my grammar text out and look up why it fell out of usage.
My kids last year (in class) were freaked that I never corrected THAT usage, and pointed out what it really stood for and actually let them use it, if used correctly.
That sure took the fun out of the word for them.
