I don't know how many of you are science dorks like me, but the Spirit Rover most likely went into "Hibernation" mode:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-106
It's funny because I've been so into that story and they've been such an unbridled success that I actually feel sad about them getting stuck and slowly deteriorating. Here's a comic about it:
The Mars rovers were supposed to last for 90 days and have lasted like 7 years. It's a great story.
