'Be' is also used in many UK dialects where standard English would have 'am', 'is' and 'are'. This dialectal use of 'be' enters the spoken language of a wider area by way of rhymes, songs, proverbs and other quotations; for example A woman, a dog, and a walnut tree, the more you beat them the ...: Information from Answers.com (not the most politically correct of examples How about this: Morning Has Broken - Bristol City's Fans' Forum :: We're Bristol City, We Always Believe ).
This sort of "be" formed its own contractions: "I be not" -> "I bain't". I remember a sketch set in a railway ticket office that featured a long string of questions - "Be there any trains going West/East/South/North today?" And the answer, after ages spent leafing through reference books, was always "No there bain't". The final question was "Be there any trains going anywhere today?" (The answer's obvious.)
"Bain't" has never been part of the nationally accepted language.