Oh, big sigh. Good writers have been using and and but to begin sentences since Old English. (Just take a gander at how many sentences begin with and in the King James version of the Bible (starting with Genesis). According to John McElroy, in his Structure of English Prose: A Manual of Composition and Rhetoric (1885), George Campbell, the peevish, was the first to rant about this common usage in his Philosophy of Rhetoric (1776). Happy reading. (BTW, I agree with my esteemed colleague, tsuwm, in his suggesting Professor Crystal.)

Ceci n'est pas un seing.