I'd Like to Thank the Engineers, and NOT the Gorvernment

The Mars Rovers are living well beyond their projected lifespans. In fact, there have been a few times where their power has increased. The biggest factor limiting the lifetime of the rovers is the ability of their solar panels to receive enough light through the inevitable dust and grime. The only explanation for the increases in power is that the panels are somehow getting cleaner at some points. Glenn Reynolds, the Instapundit, believes it is little green men with squegees. Possibly, but that would mean the rovers would have to have a stack of the local equivalent of dollar bills. So, JPL, what's the deal?