Brothers in Arms: War Stories, from Ubisoft, has been announced for DS and is in development. There are 3 screenshots in issue 214 of Nintendo Power, but I've been unable to find them anywhere else. Even Ubisoft and Nintendo don't have the game listed on their sites, but IGN, GameFAQs, and Gamespot are on top of it. This isn't 100% official yet from what I can see, but the evidence is strong that it's definitely coming. According to the ESRB, it's already rated M. One reason I'm looking forward to this is because I've always wanted BF1942 to go portable ever since I discovered the Blueroses 3D engine for GBA, and this is as close as I can get in the not-too-distant future. My hopes are definitely up for online play, since that's what makes war FPS games so great, but supporting enough players to create enough action for such an environment would be a technical challenge.
Scanned Nintendo Power article
Gamespot article