He's a douche.
Look, I'm strongly in favor of stem cell research. It holds a lot of promise for people with terrible illnesses, most of the embryos from IVF clinics will be destroyed one way or the other, and it's silly to restrict our basis of research on the basis of a viewpoint that's strongly in the minority (and yes, I understand morality isn't a votable concept, but in this case the issue is far from clear to most citizens and the views of the polity are the only basis for judgment).
But changing his stance on this issue doesn't make First a hero. It makes him the first official candidate for the GOP nomination in 2008. Even though he's pissed off some key Republican constituencies, as we've seen on a number of issues, it's more important to win than to be pure. By early 2007, when campaigning will begin in earnest, this issue will have lost its partisan venom for Frist, but the air of compassion and pragmatism will still linger about him, hopefully (from Frist's viewpoint) obscuring the pathetic image of Frist's pandering to right-to-lifers in the Terri Schiavo case.
After having vehemently opposed stem cell research for so long at the behest of the Bush Administration and the absolutist pro-life right wing, Frist's change in position is nothing more than blatant pandering, positioning himself for post-nomination general election politics, with what he gauges to be the smallest impact on his chances of gaining the Republican nomination. That doesn't make him courageous, it makes him a snivelling, pandering douche.