From the CNN story. It seems we know the reason why Bush appointed him:
During the dispute over the 2000 presidential election, Roberts was part of a team of Republican lawyers and former Supreme Court law clerks who assisted the Bush-Cheney campaign.
(Via Minipundit)
Also I believe he recently (just before the nomination) ruled (against Geneva Conventions) to allow the Bush administration to try "detainees" in a kangaroo-style military commission.
He has a strong record as a reich-wing conservative.