Mallozzi just posted this on his blog.
Regardless of whether we got the pick-up or not, we had always planned a clean conclusion to our 100th episode, one that would hopefully leave fans satisfied yet eager for more. And that “more” will come in the form of the Stargate Atlantis movie…
interesting change when only last week it was reported the last episode would be a cliffhanger
The last episode will end on a cliffhanger
http://gateworld.net/news/2008/08/iatlantisi_finale_is_enemy_at_th.shtml
Maybe it will be a little bit of both, like "Lost City." It was designed as a series finale and yet still had some cliffhanger aspects-- Col. O'Neill was on the verge of death and put into stasis at the end and Daniel Jackson realizes that they still haven't found the Lost City, merely another Ancient outpost.