From NPR -
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.
Stevens is a liberal, and Obama will certainly name a like-minded successor... thought to be on the list is Homeland Security Secretary Janet Napolitano...
Let the screaming begin. :)