Crashdown.com now has heard from several separate (and reliable) sources that the UPN has cancelled Roswell. We are hearing that the order has been cut from 20 to 18 shows for this season, and that there will not be a season 4. The cast was informed today of this decision, and public release of this information is expected at any time. Production is expected to wrap in mid-March.
Crashdown will follow this story and post any updates as we have them. If official word comes down, we will attempt to make that known asap, but at this point, it seems pretty likely that this information is correct.
It is too early to say what the possibilities are for another network to pick up the show.