Well that was fast --- as Jonathan notes below, it seems that Justice Souter is going to retire at the end of the Term. This NPR story suggests that Souter's retirement is known because he has notified the White House of his plan, which would give the White House time to nominate Elena Kagan someone in time for the nominee to be confirmed and up and running before the new Term starts.

  I should also say, kudos to Justice Souter for how he went about the process (at least as best we can tell from what we know). He didn't hire clerks just to try to keep speculation down, and he didn't wait until the last day of the Term to notify the White House. A class act.