The gossip is running wild on the web tonight and 3 scenarios are forthcoming.
1. NASA has found Fossils.
2. NASA has found Sub-Surface Water
3. NASA will say Opportunity sits in a Ocean Bed
I would say Item #3 is my best bet if you look at the pictures from Opportunity after she climbed out of her hole it appears that the Rover is is in the flattest Desert I have ever seen.
Be sure to tune into the press conference. [marsrovers.jpl.nasa.gov]