The Spirit rover is still plugging away on Mars and has nearly left the Gusev crater. The preliminary overall picture of the geology near the Spirit landing site is emerging. Most of the layered rocks appear to have originated as volcanic ash, with very prominent water action pulling out the salts from the ash. The rover might be able to figure out if the water erupted with the minerals or if it was supplied separately.
On the whole I think this region is not so promising for essentially fossil hunting. What you would have wanted to see would be a nice layered sandstone. Volcanic ash itself is very likely to be pretty sterile, so all the water in the world couldn't help something live there. Still, a lot hangs on where exactly the water came from.