Check me if I'm wrong, but didn't Stevens say something about being appointed by a Republican, so he would retire under a Republican?
I might be wrong on this, but I definitely remember reading something of the sort.
This from wikipedia:
"His possible retirement is a highly debated topic in legal and political circles in the U.S. Justice Stevens would be almost 89 years old if he remains on the court until the end of Bush's term in 2009. There have been widespread rumors on Capitol Hill that Justice Stevens plans on retiring after the midterm elections in November, 2006. Other rumors state he is planning on retirement upon the next presidential election as well as waiting to see how Bush's first two appointments shape the court. Also he has hired law clerks to last him into 2008."
I think he has said that he will wait to retire until there is a president that is more in tune with his constitutional ideology (read liberal). I think he hates Bush and will try to wait until Bush is no longer the POTUS unless his health fails him.