NASA Administrator Sean O'Keefe will be making opening remarks - something he does not typically do for scientific findings, including the water evidence. I had thought this might mean that some evidence was found of fossilized life, but one of the scientists presenting is a sedimentologist and the announcement panel - so far - does not include a biologist.
UPDATE: Seems that Opportunity is sitting on an ancient beach! From Space.com:
We think Opportunity is now parked on what was once the shoreline of a salty sea on Mars," Cornell University's Steve Squyres, principal science investigator for the Mars rover mission, said in a statement provided to SPACE.com prior to a press conference today.It's not fossils, but it's wonderful news!
The rocks would be excellent preservers of biological signs, if life ever existed on Mars, Squyres said. That makes Meridiani Planum a prime target for future missions that would search for evidence of past biology.