The only good thing here is that it increases the chances of Glenn Close and William Fichtner being appointed to the two vacant seats.
ETA: Though seriously, I can't see how this is really that bad. We knew he wasn't going to last until the next Democratic president, and it will be hard for Bush to nominate someone further to the right than Rehnquist. And it doesn't really matter who becomes Chief Justice, as that role doesn't seen to convey any additional power. I'm still more worried about the shift to the right from O'Connor to Roberts than whatever will result from Rehnquist's seat.