I suspect he stick it out until there is a Democrat in the Whitehouse. If he does it now (assuming the Dems do win the Senate), Bush still picks his successor and Bush is still likely to pick someone very conservative. It's gotten much harder to "Borque" a Judge based solely on ideology, so I suspect whoever Bush appoints in the end would still get approved, although they might not be quite as conservative as they would be under a Republican controlled Senate.
Pat Leahy is going to put the brakes on anyone to the right of Ginsburg, so there is no way a Roberts or Alito gets through. Most likely, it will be a trial and error type of thing, with at least one nominee getting shot down IMO.
Then again, he's hung on for this long, why bother? He's kinda reminds me of this very old guy I used to work with - he was well past retirement, but kept working. Somebody asked him why he didnt just retire, and he replied, "Because then I'd have to spend all day with my wife." Being a SC justice is a much better gig than moving the lawn, etc.
I also read something about this rumor, but Clarence Thomas was the one that missed Oral arguments today on some abortion cases. But Stevens is like 85 or so years old so it wouldn't be surprising if he retired soon due to old age, health problems or death.