EDIT AGAIN: The Science rumor wasn't correct either. Go here for more info.
EDIT: Here's a reason for the hush-hush about this news -- according to Leonard David, they're trying to keep whatever it is quiet so that it can go through peer-review and publication in Science. That makes sense; it's also a case in point of how terribly hard it is to maintain the old-fashioned strict embargo in the era of the Internet. Science gets published on Fridays, with news releases going out on Thursdays, but sometimes breaking stories get released first online on other days of the week. I'll keep you posted as I hear more!
The Internet is buzzing with rumors of some discovery made -- but not yet announced -- by the MECA team on Phoenix. MECA is the instrument that includes the Wet Chemistry Laboratory, the Optical Microscope, and the Atomic Force Microscope. It and TEGA form the two main chemical analysis tools on Phoenix, the instruments that allow it to study what the north polar soils tell us about present and past Mars climate and chemistry.
I don't have any more information on these rumors than is available elsewhere on the Internet. To summarize briefly: there is apparently some result from the Wet Chemistry Laboratory that is very interesting, having to do with the "past habitability" of Mars. This instrument can't discover life on Mars, so it's not that; it would have to do with the potential for life to exist, something revealing that there is some Earthlike chemistry happening now or in the past that has not before been seen on Mars. According to Craig Covault of Aviation week, it was interesting enough to brief the President's Science Advisor; however, the Phoenix mission's Twitter feed denies this.