According to the recent report by Malin et.al. if the entire southern polar ice cap melted if could raise the atmospheric pressure on Mars by a factor of 10:

MOC Observes Changes in the South Polar Cap: Evidence for Recent Climate Change on Mars
MGS MOC Release No. MOC2-297, 6 December 2001
"Mars is changing, and it is changing on a time scale that we can measure and observe. If all of the carbon dioxide that is being released into the atmosphere from the south polar cap is not freezing out somewhere else, and if it is not being adsorbed into the martian soil, then it must be causing the atmospheric pressure to increase. If this is so, and if one were to assume that the entire known volume of the polar cap is made of carbon dioxide that sublimes at the same rate we see today, then it could increase the martian atmospheric pressure by as much as 10 times, to about 1/10th the density of Earth's atmosphere, in just the next few thousand years. Although this atmosphere would not be breathable, carbon dioxide is a "greenhouse gas" that would cause the global temperature to increase considerably and make it easier for liquid water to persist elsewhere on the planet. Perhaps, just perhaps, a thickening martian atmosphere would eventually make it easier for people to live on Mars."
http://www.msss.com/mars_images/moc/CO2_Science_rel/

However, it should be said that this interpretation of the images is controversial. It may still be that the observed melting of the south polar cap is a temporary phenomenon accelerated by the global dust storms.
Also there is good reason to believe the south polar cap is not entirely CO2 but has an H2O component. However, H2O is an even better greenhouse gas than CO2 and would be expected to increase surface temperatures even further.

Bob Clark