Libby: A juror has been excused because she apparently was accidentally exposed to a news report about the trial. Details here.
As best that I can determine, the judge has decided to allow the eleven remaining jurors to decide the case, instead of starting all over, with an alternative juror joining them. That's significant because the jury's deliberations are not going to continue interrupted-- rather than having to begin "anew"-- to borrowl a phrase from Scooter Libby.
Why did Fitzgerald and Wells agree to go with eleven? Speculation from Jane. NRO has these ideas.
Update: NYT story just posted.