There were Linux server and client versions of America's Army up through v2.5 Direct Action. At that point, however, Ryan Gordon had abandoned his AA client porting work to Linux and Mac OS X. This decision was made as the US government had stopped paying him to do the porting work. Since then, all has been silent of America's Army game-play on Linux.
The release though of America's Army 3.0 is approaching quite quickly. Most notably this release will adopt the Unreal Engine 3, the game engine used by Unreal Tournament 3. It's been over 9 months without UT3 Linux client support but all hope has not been lost yet and Epic Games still assures us that this port is coming.
Apparently once America's Army 3.0 is released there is the possibility they may reconsider their abandonment of Linux and Mac OS X client ports. One of the America's Army developments has mentioned on their forums that "I've been looking into this and there are ways to make it work. We are exploring that possibility." This was in regards to an updated Mac port. However, a Phoronix reader that has spoke with one of the other AA developers has also informed us that they're interested in a Linux port too, but they currently don't have any funds allocated.
We sincerely hope that they'll be able to return to producing Linux ports of this free game, but right now we're waiting to hear back on all of the details involved. If you'd like to voice your opinions to the game's developers about these ports, you can do so in the America's Army Forums.