SimMars was to be a follow-up of sorts, to SimEarth, in the vein of the Red Mars, Blue Mars, Green Mars trilogy of books. Taking place starting in the near future, SimMars would begin with unmanned exploration of the planet, then manned exploration, and finally graduating into terraforming, and starting colonies.
With a fully 3D Engine, and support from NASA, SimMars looked to be a remarkable new paradigm in gaming. However, in recent years, information on the game has seemed to have dried up. Maxis, has instead chosen to continue working on expansions for The Sims. Neverless, SimMars is still embedded in the minds of Maxoids everywhere, thanks to the preview video on the SimCity 3000 CD-ROM, and the presence of the SimMars Testing Facility within the game proper.
It seems unlikely that SimMars will ever come out. It has been nearly five years since the news came out of its development, and nothing has come of it but screenshots and long archived news posts. Even its website has dissappeared. Shamefully Maxis did not even have the courtesy to announce the end of SimMars, like it did with Simsville. It is possible that SimMars is still in development, but with even less news on it than Duke Nukem Forever, it seems unlikely. It's just a shame.