Even if Stevens were to retire today (which there is no indication that it will be that early) ... Obama will have to go through a Republican Senate with his next nominee. It would be very difficult to push someone through before November, particularly since we can always filibuster until then.
SnakeDoc
I don't think the Repubs could maintain the unity necessary to filibuster a vote on a USSC nomination. Collins, Snowe or Lugar would peel off. Luger has said many times he refuses to block an otherwise competent Presidential appointment on ideological grounds.
Do the Senators who lost elections retire in November or January? I think there is a lame duck session from November until December 31st or the first week of January. It really won’t matter if the dims were voted out because they can vote till the new congress is sworn in in Jamuary. He can still push a justice through by then if he has all the dems and one or two pubbies (Graham and McCain). (sad to say)