See also William Connolley's personal blog Stoat, and links there.
Anyway, let us return to the Martian climate:
Mars has a relatively well studied climate, going back to measurements made by Viking, and continued with the current series of orbiters, such as the Mars Global Surveyor.
Complementing the measurements, NASA has a Mars General Circulation Model (a global climate model), run out of NASA Ames.
There is a good "general reader" review of modeling the Martian atmosphere by Stephen R Lewis (Oxford) in Astronomy and Geophysics, volume 44 issue 4. pages 6-14.
In 2001, Malin et al published a short article in Science (subscription required) discussing MGS data showing a rapid shrinkage of the South Polar Cap. Recently, the MGS team had a press release discussing more recent data showing the trend had continued.
MGS 2001 press release
MGS 2005 press release
The shrinkage of the Martian South Polar Cap is almost certainly a regional climate change, and is not any indication of global warming trends in the Martian atmosphere.
Colaprete et al in Nature 2005(subscription required) showed, using the Mars GCM that the south polar climate is unstable due to the peculiar topography near the pole, and the current configuration is on the instability border; we therefore expect to see rapid changes in ice cover as the regional climate transits between the unstable states.
Globally, the mean temperature of the Martian atmosphere is sensitive to the strength and duration of hemispheric dust storms, (see for example here and here). Large scale dust storms change the atmospheric opacity and convection; as always when comparing mean temperatures, the altitude at which the measurement is made matters, but to the extent it is sensible to speak of a mean temperature for Mars, the evidence is for significant cooling from the 1970's, when Viking made measurements, compared to current temperatures. However, this is essentially due to large scale dust storms that were common back then, compared to a lower level of storming now. The mean temperature on Mars, averaged over the Martian year can change by many degrees from year to year, depending on how active large scale dust storms are.
This however should not be taken as evidence for a secular global cooling trend, or change in solar irradiance!