Yeah, Mars has lots of little dust devils...they could have blown the dust away.
Wonder why NASA just didn't add a little mechanical brush above the cell panel and use it every so often. Weight vs. cell efficiency...I'd bet they could've come up with some kind of lightweight system. Oh, well. Maybe next time.
Funny you should mention that. On Futurama last night, the Mars Natives (aka, Indians) were calling up dust devils....