From my dictionary:
usage: The use of ain't was widespread in the 18th century and is still perfectly normal in many dialects and informal contexts in both North America and Britain. Today, however, it does not form part of standard English and should not be used in formal contexts.
Ain't is absolutely proper English when used in the right place. As the usage note states, it should not be used in formal language, but if you are talking with friends and one of them asks you to lend him $20 it would be fine to say "I ain't got $20".