That's right, Jon.
It should be remembered that there is still the evidence for
earlier, less acidic (and less salty?) water, which left behind the clay mineral deposits. Ray Arvidson of the MER team mentioned this again in this recent December 10, 2007 update:
http://marsrovers.jpl.nasa.gov/newsr.../20071210a.htm
"We see evidence from orbit for clay minerals under the layered sulfate materials," said Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the rovers' science payload. "They indicate less acidic conditions. The big picture appears to be a change from a more open hydrological system, with rainfall, to more arid conditions with groundwater rising to the surface and evaporating, leaving sulfate salts behind."
ALL of these observations must be considered...