Dust Devils!
They've gotten better at catching them:
Enhanced image, I'm afraid. It's still easy to see on the raw frame, but not as impressive.
Spirit caught devils on two recent days, in navcam sequences that allowed the team to put together animaged gif "movies" -- the devils are only visible for a few frames, so there's not much motion, but still cool.
Apparently they are now taking navcam sequences over a short period, sending down the thumbnails to see if anything changed in the frames, and only downlinking the full images if the sequence captured something.
Press release:
http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20050421a.html