So it was a big announcement - but not a terribly surprising or incredibly dramatic one. Indeed you might argue that it was pretty close to being the smallest discovery that would still merit a big press conference at headquarters. Doesn't mean the news wasn't important or exciting. Just means -- and this is no surprise -- that speculation can outstrip the science pretty easily.
Here's the gist, with a bit of my take.
What they now know for sure:
1) There's clear evidence in Meridiani Planum of rocks that were soaked - "Drenched", I think Ed Weiler said - in salty water some time in the past. This means there was once a habitable environment there. Habitable, here, means earth bugs could have settled down and done there stuff. Many lines of evidence for this, but the clincher seems to be a rare (on earth) hydrated sulphate called jarosite. If I heard correctly (and real audio seemed to be choosing all the best moments to cut out in) jarosite on Mars was a favoured speculation of the veteran USGS astrogeologist Henry Moore, who died a few years ago. It's also been studied by people using Hawaii as a Mars analog, which will doubtless please Jeffrey Bell (see this post, near the end)
2) There's a record in the rocks not just of wetness but also of environmental change (change in the local environment, that is - not necessarily anything planetwide). Holes in the rock seem to mark places where mineral crystals grew and then dissolved, suggesting a change in the chemical environment. Ben Clark of Lockheed - one of the few Viking veterans on the science team - pointed to evidence of a drying-out process captured in the sequence of different elements in different layers of the rock that's similar to the sequences found in evaporites on earth.
3) That by using all the instruments on the rover together, it really is possible to make definitive, if fairly circumscribed, discoveries about Martian geology. (Obviously, that was always assumed - but it's very nice to see it being proved. Indeed, this meta-discovery may be the most momentous discovery so far.)
What they still don't know, but might find out:
1) Whether the water was sitting on the surface (either ice-covered or not - see the previous post), or subsurface water percolating through rocks.
2) Whether there's more neat stratigraphy to be found above or below this sequence.
3) Whether there's evidence for more recent brines at either of the sites (a lot of speculation about this before the press conference, no mention during.)
4) Whether there are fossils in the rocks. Unlikely, but not inconceivable - if the site was habitable, it could have been inhabited, and if it was inhabited some sign might have been preserved. If there are fossils, though, in order to be recognised as such they'll have to meet the Knoll criterion, named after Andy Knoll, the expert on fossilised earth bacteria who's working on the mission. The Knoll criterion is that anything being put forward as a fossil must not only look like something that was once alive - it must also not look like anything that can be made by non-biological means. (Andy's book on the precambrian earth, Life on a Young Planet, was one of the best science books I read last year; a real expert explaining his field in some detail, both carefully and elegantly.)
What they don't know and won't find out on this mission
1) How long ago this all happened. (But we know it was a long time. Meridiani Planum is a pretty old surface; and as Steve Squyres said, "There's nothing like this happening today")
2) How long this wetness lasted.
What happens next
1) Spirit keeps on rolling towards Bonneville crater.
2) Opportunity hangs around in its little crater for another ten days or so, looking for more of the evaporite sequence, looking for cross bedding in the layers (which might show whether the layers were laid down underwater, in the open air, or in some sort of surge of volcanic gas) and trying to work out what the little spherules (the blueberries) are. Apparently individual blueberries are too small for the spectrometers to do much with, but a little claque of blueberries has considerately rolled down into a dip that's being called the blueberry bowl, which means the instruments may be able to get a good grasp on their average composition. After all this, Opportunity heads out to a crater being called Endurance, about half a mile away, which it might reach in ten days or less.