This podcast episode had a few things about homebrewers going pro, including a comment, by Brewers Association director Paul Gatza, that 90% of professional craft brewers started as homebrewers. IIRC, the same statistic was mentioned by Ken Wells, of Travels with Barley fame on the July 16, 2005 episode of The Beer Show. This statistic must be listed somewhere on the Brewers Association website.