Like a sudden upsurge of power during the day (surely telemetry would reveal that), or (unlikely I guess) before and after pictures?

Part of the reason the estimated life of the rovers was so short was that they expected that the dust to 'cling' to the solar panels tightly... apparently they were wrong- (and that is great news).

Now, I would have expected the dust to cling to the panels for the following reason (This may not have been the reasoning behind the rover team's pessimistic prediction):

During the day the rover is bombarded by ionizing UV radiation, this could well lead to a static charge on exposed insulated surfaces- such as solar panels. dust would adhere to these surfaces and cling tight. (Blow a handful of talc onto your TV screen while its on, for a demonstration). The panels would be positively charged as a result of photoionization.

But at night the ionizing radiation is no longer present, so static charging of the panels would cease- unless there is some process for quickly neutralizing the panels then the 'static cling' will remain and only slowly dribble off... the strength of the 'cling' probably wouldnt diminish enough for ambient winds to clean the panels...

BUT, what if there is a mechanism that speeds the neutralization of the positive charge on the panels...? In otherwords a source of negative charges?

Please look at: http://sprg.ssl.berkeley.edu/matt/mars/mars_gec.html

A section worth noting (although the entire article is worth a good read):

The absence of ozone in the atmosphere of Mars allows photons with energies up to 6 eV (~200 nm and longer) to reach the surface. The work functions of many of the Martian surface minerals are between 4 and 5 eV [Grard, 1995]. Therefore, the surface minerals will emit photoelectrons due to incident solar ultraviolet radiation. Grard [1995] suggested that these free electrons attach themselves to ambient atmospheric molecules via a three-body process creating negative ions. This negative charge distribution above the positively charged surface yields an upward electric field at the surface on the order of 10-2 to 10-1 V/m by his estimates.

So during the day a large amount of atmospheric negative ions are created near the surface of Mars, these negative ions would likely be constantly serving to fight the positive charging of the rovers panel, but during the day the charging would likely dominate the trickle of negative charge supplied by the ions (as this paper seems to indicate it does when considering the Martian surface). But as the sun sets this will cease to be true- At some point the flow of negative charge from the atmosphere will dominate the creation of positive charge in the panel surfaces.

At this point the rover has a dominant source of negative charge to return its surface to neutrality... This return to neutrality may make it easier for ambient winds to remove the fine dust...

If this mechanism is the culprit then the 'cleanings' will only happen at night... so the hypothesis is somewhat testable...

But from the same article-

This vertically upward electric field would be rapidly shielded if there was not a continuous source of negative ions to the atmosphere supplied by photoelectrons created by solar ultraviolet radiation. Therefore, this effect is only important on the day side; this effect rapidly decays after sunset.

So the question is, how rapidly does this resevoir of negative ions decay after sunset and what is their initial density?