Regarding the resignation of Supreme Court Justice John Paul Stevens, it's now a matter of when, not if. Weeks away from his 90th birthday, Stevens is the court's senior justice in both terms of age and service; he was named by President Ford in 1975. An official announcement is expected soon.
Part of the calculation is of course politics. Stevens has made it clear he wants President Obama to name his successor. If it's this year, it will once again turn the Senate floor into an ideological foodfight, similar to what we saw during the battle over health care ... only this time it will be in the middle of the midterm elections. And Obama still is hoping for GOP cooperation on issues like climate control, immigration and financial regulation. Sen. Arlen Specter (D-PA) said on Fox News Sunday that it would be better if the confirmation debate happened next year, after the November elections, when tempers might be cooler. But the number of Democrats in the post-election Senate could be smaller as well, which has to be part of the calculation.
Stevens is a liberal, and Obama will certainly name a like-minded successor; the names most commonly mentioned are federal appeals court judge Diane Wood of Chicago (whom I thought Obama would pick to succeed David Souter last year when he instead chose Sonia Sotomayor), Solicitor General Elena Kagan and DC appeals court judge Merrick Garland. Also thought to be on the list is Homeland Security Secretary Janet Napolitano, and Michigan Gov. Jennifer Granholm has come up as well.
Replacing a liberal with a liberal wouldn't affect the ideology of the court, but that didn't stop 31 Republicans from opposing Sotomayor. So it's probably safe to assume that there will be a nomination fight no matter whom Obama chooses.
The last instance of when a court nominee was dramatically different in ideology from his predecessor came in 1991, when Clarence Thomas was chosen by the first President Bush to replace Thurgood Marshall.