MarsLab at Cornell - The Rovers Just Keep Going and Going and Going...
"The rate of dust buildup has slowed," he noted, "and when you combine that with the fact that we've gotten clever about parking these things on north-facing slopes where the sunlight is stronger, we've found that we don't need recharge sols. Both rovers have been active throughout the Martian winter, and soon we'll start to see spring coming on."
http://www.news.cornell.edu/Chronicle/04/11.11.04/MarsLab.html