I remember watching NASA TV and this question was asked. The answer was something to the effect that the particles that acumilate are tiny "dust" and it would take something like a liquid and a squeegie(sp?) to clean them, and even then not completely. The cost of inventing and creating a devise that would work (liquid would freeze instantly so this idea would not work.) would be too costly and time consuming. This mission was to perform a few simple tasks, setting up further missions to do more. Anyway, the rovers are going just fine and they are way over the targeted lifespan projections and may go on for more than anyone would have ever expected. By the way, they have really done what they went there for, and all they will continue to find with the instruments they have will be more of the same so really prolionging their life wouldn't benifit anyone much.

The rovers were only supposed to go 90 days to meet the requirements. The dust is very fine, like corn starch or diatenacious earth, not like sand. The dust there also appears to develop a static charge. They are looking at putting the rovers into hibernation for the winter and see if they can revive them next summer.