This seems to have been in the works for longer than you think, but it is pretty coincidental that he's announce it two days after the Republicans lost the filibuster.
Lets posit that McCain was the President. THere’s no way in hell Souter would have retired. Although maybe with they’re only being 40 republicans he’d have have figured that McCain would have had to appoint a moderate anyway.
Ah, what am I saying, even if McCain or even Bush was the President, there’s no way we’d get a conservative confirmed with only 40 Republicans so it doesn’t matter anyway.