Whats the premise ?
Information is scarce right now, but we do know that the upcoming feature film will establish the premise for Atlantis. Executive producer Robert C. Cooper has said that the film "evolves the Stargate."

Fans have also begun to theorize on how the myth of the lost continent of Atlantis may be incorporated into the Stargate universe. One prevailing theory is that the film will introduce the Ancients, the builders of the Stargates, and identify them as the original inhabitants of Atlantis. (Read our Gatewatch editorial for more on that theory).

So that it differs substantially enough from its predecessor, Atlantis will probably not be set in the SGC -- and may very well not be set on Earth. Creator Brad Wright told fans that it will not be based on a military team.

Who will be in it ?
Casting has not yet begun for the show, and it's unknown which characters might appear in the new series. Most likely, it will be a new cast and a new set of characters. While SG-1 cast members could make guest appearances, don't count on any of the show's regulars to sign on to be regulars on Atlantis.

When will it air ?
Stargate SG-1's seventh season concludes filming in the early Fall of 2003. If it turns out to be the show's final season and the crew heads straight into production on the film, don't expect Atlantis to bow before the fall of 2004.

Where will it air ?
There's no word yet on which network might air the show, though the fact that it is coming has already been advertised to the syndication market in the U.S. (where SG-1 currently has its second run, following Showtime / The Sci-Fi Channel). SG-1's record-breaking success on Sci-Fi bodes well for Atlantis possibly airing there when SG-1 ends.