The news release from the AGU conference argues that the Mars "spider" features at the south pole are due to gas release:

HiRISE | Isolated Araneiform Topography (PSP_003087_0930).
http://hirise.lpl.arizona.edu/PSP_003087_0930

Spring at the South Pole of Mars.
AGU Press Conference
C. J. Hansen, A. McEwen and the HiRISE Team.
December 2007
http://hirise.lpl.arizona.edu/pdf/agu_press_conf_dec07.pdf

However, this report shows temperatures above 250K can be reached seasonally even as far south as the Mars spiders:

Astrobiology
Spiders: Water-Driven Erosive Structures in the Southern Hemisphere of Mars.
Aug 2006, Vol. 6, No. 4: 651-667
http://www.liebertonline.com/doi/abs/10.1089/ast.2006.6.651 [abstract]

This is within the temperature range of calcium chloride liquid water brines.
Evidence for a liquid water origin is the dendritic pattern shown by the spider "legs". On Earth this is taken for evidence of channels cut by liquid water.
The news release argues that the channels are cut by gas because some channels travel uphill. But of course water can also travel uphill for short distances under pressure or simply because it is traveling quickly as in a flood.

A related possibility is that the origin of the spiders is due to the transition of CO2-water clathrates to liquid water brines. This report shows this could occur on heating and would result in explosive gas release:

PTX PHASE EQUILIBRIA IN THE H2O-CO2-SALT SYSTEM AT MARS NEAR-SURFACE CONDITIONS.
R.J. Bodnar1, 1Fluids Research Laboratory, Department of Geological
Sciences, Virginia Tech, Blacksburg, VA 24061 email: *...@vt.edu.
Lunar and Planetary Science XXXII (2001) 1689.pdf
"The decomposition of clathrate to produce brine
plus gas would generate considerable mechanical (PV)
energy owing to the large volume change associated
with this reaction. The reaction of clathrate dissociating
to produce gas plus brine is analogous to the process
that occurs in silicic volcanic eruptions on earth,
whereby water dissolved in the melt exsolves to form a
separate fluid phase. The large difference between the
molar volume of the exsolved fluid (water) and the
partial molar volume of water dissolved in the silicate
melt provides the energy for explosive volcanic eruptions.
A similar process associated with the breakdown
of carbon dioxide clathrate on Mars may provide
the energy to move large volumes of regolith material
to produce the characteristic erosional features that are
observed on the martian surface."
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1689.pdf

The plausibility of either of these explanations is easy to test experimentally in the lab. For the gas-carved only explanation the key questions is would such gas carved channels show the dendritic patterns usually seen only with liquid water carved channels?
For the dissociating clathrate explanation, the key question is would the clathrates dissolving under a CO2 ice cover result in liquid water brines at sufficient pressure to carve channels that even travel uphill for short distances?

Bob Clark