For instance (thanks for the links) -

.. looking at the navcam shots, the solar panels either have hardly any dust coating or else a coating too sparse to interfere with power needs. From this Planetary Society link:

Perhaps even more interesting is the fact that the dust accumulation on the rovers' solar arrays isn't the problem the team thought it might be at this point. "If the dust accumulation had continued at the rate it had been going, we would have enough solar energy to last until May or July '05, however the dust accumulation has stopped increasing," Adler said. "We figured sooner or later it ought to level off -- there ought to be some equilibrium reached between the dust accumulating and the dust being blown off -- and we think we might be there now, in which case energy may not be the limiting factor."