You mean, other than O'Connor, who has already said she's retiring?You're inferring another vacancy is likely, so who's gonna snuff it or retire?
I'm not as concerned about Roberts being a chief justice as I am whomever is going to be nominated to O'Connor's spot.
Alberto Gonzalez has been a name in circulation, and my bet is that he's even more conservative than Roberts. Bush wouldn't have re-nominated Roberts to Chief Justice unless Roberts was likely to be confirmed as Chief -- meaning, he has to appeal to both Democrats and Republicans alike. He's under a finer microscope than the other Jusice that Bush nominates will be.
So, with all that in mind, I think the most conservative Justice appointed by Bush will be the one that he names NEXT -- not Roberts.