Mars is a hot subject right now, and perhaps this announcement got lost in the shuffle. UCLA scientist An Yin put out a press release early this month announcing he discovered compelling evidence of plate tectonics on Mars. The left lateral strike-slip fault shows clear signs of movement, with part of an impact crater severed. For a good picture of the fault from space, check out this news post on Discovery.com.
After some digging, I actually managed to find the scientific paper that details this study (unfortunately it is not available at the ANU Library). Valles Marineris, the feature in question, is over 2000 km long, and had more than 100 km of motion along strike. In comparison to faults on modern Earth, this is not that great, but it may be comparable to faults on Earth during the Archean. Mars is much smaller than the Earth and probably did not store as much heat, so tectonics may have been restricted to this small area of the planet. Yin plainly calls the tectonic plates “Valles Marineris North and Valles Marineris South”. This is a pretty interesting change of interpretation for a feature that was initially thought to be caused by erosion or rifting.