I don't think Ginsburg is going anywhere for a long time. (If only the GOP hadn't confirmed her.)
Well I don't think she'll leave before the '08 elections for sure, but she does have lingering health problems. When a conservative president is elected in '08 there's no way Ginsburg can hold on long enough to avoid being replaced.
Stevens is most likely to leave first because of his age and statements saying he wanted to retire under a Republican president.