I'm not sure if it has been mentioned here or not, but Nick Hoffman is a co-author of a soon-to-be-published paper in Geophysical Research Letters. The applicable EurekAlert press release describing the results can be found at:

Martian surface features were eroded by liquid carbon dioxide, not running water, researchers say
http://www.eurekalert.org/pub_releases/2002-03/agu-msf031202.php

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FWIW, I've had the opportunity to read a preprint and find the paper to be very interesting. Tanaka et al. provide a very compelling case (based on MOLA data) for catastrophic erosion at Malea and Hesepria Plana (on the rim of Hellas Basin), and they further allude that future tests (perhaps cm-scale orbital imaging of clastic sorting in the debris flow path) *might* help to distinguish the volatile(s) in question (H2O, CO2, or a combination of both). Though I'm am an advocate of H2O-based models, I have to admit, in fairness, that Tanaka et al. do show that CO2 or CO2-related processes (at least in this area) are, as they state, "viable," "alternative" models to the H2O models (e.g., glaciation, ice-dam outburst flooding, etc.).

Alex R. Blackwell
University of Hawaii