See in particular the most recent paper from NASA (November 2009):
Life on Mars: New Evidence from Martian Meteorites
"New data on Martian meteorite 84001 as well as new experimental studies show that thermal or shock
decomposition of carbonate, the leading alternative non-biologic explanation for the unusual nanophase
magnetite found in this meteorite, cannot explain the chemistry of the actual martian magnetites. This leaves the biogenic explanation as the only remaining viable hypothesis for the origin of these unique magnetites. Additional data from two other martian meteorites show a suite of biomorphs which are nearly identical between meteorites recovered from two widely different terrestrial environments (Egyptian Nile bottomlands and Antarctic ice sheets). This similarity argues against terrestrial processes as the cause of these biomorphs and supports an origin on Mars for these features."
