This announcement got very little attention: the Mars Observer satellite found clays in the rim of the Endeavour Crater, which is the distant destination of the Opportunity Rover. While the rover has already analyzed water-deposited rock, the rock was from highly acidic water that is less hospitable to life than the earlier, phyllosillicate-clay rock found elsewhere. A major reason for the delayed Mars Science Lander mission is to examine clay rocks, and this discovery could give a preview.
The sequence of events as I understand it is that scientists believe that clay rock layers were deposited first. Then a large impact created the Endeavour crater, with crater rims sticking up above the surrounding rock. Then acidic waters left acidic rock deposits that covered the entire area except the crater rim. If Opportunity can get to it - it's still a long way away - it can find a type of surface more life-friendly than anything we've seen before.
I've wondered if the $20m annual cost of extending the rover missions has been worth it, but this would make it worthwhile if the rover successfully gets there.