There's constantly rumours that Roland Emmerich will make another but hopefully it won't come to pass. I think the original is brilliant but I feel it would be a bit of a throwback now to go back to that storyline.
With regards to the tv films it doesn't look like they're going to happen either. I remember when Atlantis was canceled one of the conciliatory offers made by the studio was that Stargate: Extinction was going to provide a fitting end to Atlantis.
MGM is in a heap of trouble money wise at the moment though I believe so all of the tv show movies are on hold indefinitely which is a shame.
Would be a bit silly for Emmerich to ignore the series and do another movie.
As for the tv films, the production staff are all very much set on doing the films, they're just still waiting to hear stuff from the studio. I would never say never, the fact that MGM is in allot of trouble with debt as has been mentioned has kind of put a very large spanner in the works.
If you can get past the constant obsession with dog and food pictures then JM's blog is pretty good for insight into SGU and any happenings on the SG1 and SGA movies (You can be sure that if there is any news on the latter it will be there pretty sharpish).
http://josephmallozzi.wordpress.com/2010/04/06/april-6-2010-sgu-production-update/