You think John Paul Stevens will step down

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.