Firefall is an upcoming co-op shooter for the PC, made by Red 5 Studios, which is helmed by the guy who was team lead on World of Warcraft. So it should know what it's doing when it comes to online gaming.
It certainly looks like it does, as this gameplay trailer is a blast. If you've ever liked Starship Troopers and/or Tribes, there should be something to like in all this.
It should be out sometime towards the end of the year.