UPDATE: Stephen Gorevan of Honeybee Robotics emails:

The article you linked to about JPL’s missions used the Spirit and Opportunity rovers as the prime example of mission life understatement. But many observers forget that the 90 day lifetime for the Spirit and Opportunity surface ops was based mainly on a prediction that the solar panels on the rovers would take on enough wind blown dust such that the rovers would no longer be able to recharge their batteries. We did not know the Martian winds and dirt devils would regularly clean the panels enabling years of battery life. I know this to be so not only because I am a member of the MER science team but also because my engineering company had a contract to devise a mechanism that would clean off the solar panels for future missions. No one knew such a device would not be necessary!

Very interesting.