Scalia would be wheeled into court on a gurney rather than step down during an Obama Administration. Unless he dies, he ain't goin' anywhere.
Thomas is not even 65, he's not going anywhere for a while. If Conservatives have to worry about anyone, its probably Kennedy - he's in his mid-70s and has no real idealogical conviction to ensure the Court goes in a certain direction once he's gone. But, he's hardly reliable anyway, although far better than what you'll get if Obama has the opportunity to replace him.