EDIT: Here's my longer writeup on Spirit's transition to landerhood.
There's a press briefing going on right now that marks today, January 26, 2010, more than six years after she landed, the day that NASA decided that Spirit's roving days were over. Doug McCuistion, from NASA Headquarters, said "We do not believe [Spirit] is extractable." He said that while it's possible that future movement might "accidentally" extract the rover, with only four operable wheels it is not realistic to expect her to drive anywhere else even if she does pop out of the sand trap she's currently in.
What's ironic is that Spirit had actually been making very good progress in her last few sols of driving. But they have to focus on winter survival now, not extrication. That's partially good news, though -- although NASA thinks that JPL needs to quit trying to get the rover out of the trap, NASA also thinks that there is a lot of science value left in Spirit's future operations as a stationary lander. So it's worth making every effort now to help Spirit survive the coming winter so that she can do science in the spring.
There is much more for me to write on this, and I will try to get a post done later today, so stay tuned for more.