In light of Rehnquist's death, George Bush has decided that Roberts will be replacing Rehnquist, rather than O'Connor, who has pledged not to retire until a replacement is found. Rehnquist made no such promises, regarding his death.
So, Bush isn't choosing two new justices yet, but he's now gotten an added sense of "urgency" to Robert's nomination. There is plenty of time for debate with O'Connor's replacement...without a replacement for Rehnquist in the near future, it can be spun that the court will suffer with only eight justices on the bench. Pretty convenient, eh?
Of course, it would have been nice if Bush had waited until Rehnquist's corpse had cooled off a bit before making the announcement. Then again, Bush isn't known for his decorum.