As Howard points out, because of Justice O’Connor’s position as a swing vote on the Court, the battle over appointing her successor could be fierce. Note the opposition to Alberto Gonzales following the rumor of an O’Connor retirement.
If the Chief retires now too, then two confirmations could be sold as a package: one, more conservative, to replace the Chief, and one more moderate (or perhaps simply of unknown qualities) to replace Justice O’Connor.
If the Chief waits, there will be two separate, bitter confirmation battles, and they could take place during the runup to the 2006 elections, when things will be even more politicized.
So I would not discount the possibility of a second retirement announcement coming very soon.
UPDATE: Lyle Dennison has a different view on this question.