"The widespread public belief that 'but' should not be used at the beginning of a sentence seems to be unshakeable. yet it has no foundation. In certain kinds of compound sentences, 'but' is used to introduce a balancing statement. In such circumstances, 'but' is most commonly placed after a semi-colon, but it can legitimately be placed at the beginning of the following sentence, and frequently is."
Fowler's Modern English Usage
Willa (Tongue in cheek!)