Interesting article online in Nature News:

Published online: 27 July 2004
The search for life on Mars
"Several groups have now reported seeing water vapour coming from certain parts of Mars, such as Arabia Terra and Elysium Planitia. Other researchers believe that water ice lies under the surface at these locations.
"The presence of water vapour implies that ice under the martian soil in these locations is being melted by some heat source and evaporating into the air. The most likely heat source is volcanic, and, as we have learned from studies on Earth, where there's heat, there's life. Many extremophile bacteria live on the chemical energy found in minerals spewed out by volcanic vents under the sea."
http://www.nature.com/news/2004/040726/full/040726-3.html

This article also reports Mars Express found high water vapor over Arabia:

Probe maps water vapour on Mars.
By Paul Rincon
BBC News Online science staff, in Paris
Mars Express has detected an area of high water vapour over a region of the Red Planet called Arabia Terra.
http://news.bbc.co.uk/2/hi/science/nature/3927041.stm

Most of the reports I've seen attribute high hydrogen rates seen by the GRS instrument on Mars Odyssey on Mars at Arabia to chemically bound water, since this is an equatorial site.
The Nature News article seems to suggest the independent high water vapor measurements at Arabia are interpreted by some scientists as due to free water there, which could melt to liquid water at Summer, daytime temperatures.

Bob Clark

cf.:

From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Latest Mars Odyssey results suggest free water at equator on Mars.
Newsgroups: sci.astro, alt.sci.planetary, sci.physics, sci.geo.geology, sci.geo.mineralogy
Date: 2003-07-19 01:43:17 PST
http://groups.google.com/groups?selm=832ea96d.0307190043.2e963cec@posting.google.com