Sect. Rumsfeld tendered his resignation, along with all of the rest of the Cabinet, before President Bush was re-elected. That envelope is old, and dusty.
/johnny
That's true, but I think this was more recent. Bush has stuck by truly hopeless people, and Rummy was by no means hopeless. I think Rumsfeld simply got tired of thinking of himself as the albatross around the Administration's neck.
He wasn't, of course, but he was a particular target for the media because he was unabashedly himself, Rumsfeld, and gave them no quarter. I think he probably felt that leaving would protect Bush, although the truth is that it made Bush look even worse.