Justice Rehnquist's death at this time will, I think, show the down side of the decision by opponents of the Bush administration to gear up against the Roberts nomination.
Roberts is superbly qualified to be on the U.S. Supreme Court. His records suggests that he will not be reckless in approaching precedent, and will justify any decisions with painstaking and scholarly reference to the Constitution and prior USSC cases. He is less predictably conservative than many other basically-qualified candidates Bush could have picked to replace Justice O'Connor. Yet because Bush picked him, and because he is likely to ally with conservatives in some case somewhen, the opposition has rather visibly strained to find credible reasons to oppose his nomination.
Now that Bush has a chance to replace Rehnquist, he is likely to pick someone more obviously conservative, as Rehnquist was more obviously conservative than O'Connor. Yet for most observers not already members of the choir, the likely objections to the new candidate will sound like somewhat dubious echoes of the less well-founded objections raised to Roberts. Crying wolf is a very short-sighted strategy.
UPDATE: Obviously, I didn't predict what actually happened. Given O'Connor's promise to stay until her replacement was confirmed, it made eminent sense for Bush to move Roberts over to the Chief Justice vacancy. Bush may or may not go for a tough conservative the next time around. If he doesn't pick someone glaringly controversial, the cry-wolf analysis still makes sense to me.