The truth is that "ain't" is indeed a word. Don't believe me? It's in most major English dictionaries.
Each year about 50 new words are deemed legitmate and inducted into word-dom (which happens to be a non-word that I think will be on next year's list ;o).
I have also learned that my much loved and almost favorite non-word "ginormous" is finally being welcomed into the covers of the Merriam-Webster dictionary along with a number of other non-words which can all be viewed at http://www.m-w.com/info/newwords07.htm .
Perhaps you are wondering how a non-word earns this great honor. Well, "ginormous" did it by winning a popularity contest. It stayed in the rankings for the top ten favorite non-words for a period of years before it was considered. If you'd like to stay on top of the new terminology before it becomes terminology, you can check out the favorite non-word list at: at http://www.m-w.com/info/favorite.htm.
And for the few of you who may scoff at American English and dictionaries, you can see the new British English words in the Oxford dictionary: http://www.askoxford.com/worldofwords/newwords/?view=uk