We've done this, with minimal research...Mexico...decent results. Now we're looking to do it again after a couple of years of more teeth grinding and are thinking Turkey this time. "Dentisanbul", anyone? (Message me if you have experience with this, would 'ya?)
Oh, and apparently there is a rising market for this and other healthcare tourism. See:
http://www.msnbc.msn.com/id/25415614/