Orin Kerr thinks so. I agree!
Since O'Connor's resignation was contingent on replacement, isn't it possible to switch the Roberts nomination to Chief Justice and have a nine-member Court sitting when the term begins? Then replace Justice O'Connor.
UPDATE: Actually, I think this would be a poor strategy for Bush if his goal is to produce a more conservative Court. In that case, he should follow through with the Roberts nomination ousting the swing voter O'Connor. That is nearly an accomplished feat at this point. If he replaces Rehnquist first, the debate about how a moderate should replace O'Connor will return to square one.