Paul, You're right--I was misremembering Webster's entry on ain't when
I made that post. I owe an apology to the whole list. According to
Webster's, ain't apparently arose from contractions for "am not" and
"are not" apparently simultaneously, with differetn spellings (an't and
a'n't). It also came to be used for "isn't" and and even "have not."
        I wsa remembering this passage in Webster's:
 
"Hill...quotes the linguist Raven I. MCdavid, Jr. to the effect that
_ain't_ lost status as a pronounciation while the broader of the two
pronounciations probably represented by _an't_ gained status." --pg 61
left column, middle
        --Bill Murdick