The paper by Byrne and Ingersoll arguing for a thin CO2 cover over a H2O southern ice caps also suggests this thin CO2 cover would sublimate and reform periodically on quite short time-scales:

A Sublimation Model for Martian
South Polar Ice Features.
Shane Byrne and Andrew P. Ingersoll
"With an active slab that is
4 m thick, the outward expansion phase
begins after 30 martian years; with an
active slab that is 20 m thick, it begins after 300 martian years."
...
"The time needed to completely destroy
the upper 8-m CO2 ice slab depends on the spatial density of these depressions and their expansion rate. Taking the region named above as typical, the mean distance between depressions is 300 m. Two depressions growing toward each other, each 0.5 to 2.5 m year, will meet in 60 to 300 martian years and will remove all intervening mesas in 85 to 425 martian years. Large sections of the residual cap appear in MOC images as though the 8-m covering has already been removed. Such areas are probably covered in 2 m of CO2 ice, which in especially warm years may entirely disappear (as perhaps happened in 1969)."
SCIENCE, VOL 299, 14 FEBRUARY 2003, p. 1052
http://www.gps.caltech.edu/~shane/byrne_and_ingersoll.pdf

However, they estimate the added pressure of the entire CO2 cover sublimating would only be an additional .36 mbar. This estimate seems too low to me. Nick estimated the added pressure to be about 4 to 5 mbars:

Re: Thin CO2 cover?
Posted by Nick Hoffman (61.9.128.173) on 2/13/2003 8:40:33 PM
In Reply to: Thin CO2 cover? posted by Robert Clark on 2/13/2003 8:00:25 PM
"...The permanent polecap is less than 500 km across, so if it has 20 m of CO2 ice, that corresponds to 4-5 mBar of CO2."
http://habitablezone.com/space/messages/30887.html

which seems to be a more likely estimate. This added atmospheric pressure may have allowed open water pools within the time Mars has been telescopically observed by humans in the lowest basins on Mars such as the Hellas basin.

Bob Clark