Just saw this article on the Aviation Week site:

White House Briefed On Potential For Mars Life.
Aug 1, 2008
By Craig Covault
"The White House has been alerted by NASA about plans to make an announcement soon on major new Phoenix lander discoveries concerning the "potential for life" on Mars, scientists tell Aviation Week & Space Technology.
"Sources say the new data do not indicate the discovery of existing or past life on Mars. Rather the data relate to habitability--the "potential" for Mars to support life--at the Phoenix arctic landing site, sources say.
"The data are much more complex than results related NASA's July 31 announcement that Phoenix has confirmed the presence of water ice at the site."
http://www.aviationweek.com/aw/generic/story.jsp?id=news/WH08018.xml&headline=White%20House%20Briefed%20On%20Potential%20For%20Mars%20Life&channel=space

This apparently has to do with the MECA instrument, not the TEGA instrument used to detect water/ice at the Phoenix site.
The article says it is not organics, nor actual bacteria which the optical microscope in MECA might be able to detect.
Some guesses: maybe using the microscope, MECA was able to find clays. Or perhaps it was able to detect carbonates in significant amounts. Both TES from orbit and mini-TES on the rovers though have already detected carbonates in small amounts as dust on the surface.
MECA can also detect nitrates and dissolved oxygen, both of which might be supportive of habitability.

Bob Clark