According to Destructoid, a new Aliens DS game was in the works but was cancelled by Sega due to financial reasons. The game was a side-scroller developed by Wayforward in association with Gearbox Software.
Gameplay footage has found its way onto YouTube. Notable scenes include a Space Jockey Alien, a Rhino Alien (?), and a Powerloader fighting with a Queen. Thanks to Mr. Weyland for the news.
Update: Sega had the video removed from YouTube. You can watch the gameplay footage over on GameVideos.