Probably just a trial balloon. The position isn't even available yet. I'm sure W has his ducks in order for when vacancies become available.
Rehnquist's form of thyroid cancer turns out to be the worst kind - His time on the court is very limited, I'm afraid. I suspect he will either step down (he was holding on for a Bush reelection and now knows he can let go)- or, sadly, will be gone.