To be more specific, it's Nathan Fillion's character in Halo 3: ODST, Buck. Buck lays down pretty much everything you need to know about the game, introducing the various squad members, their equipment and their mission. Sci-fi nerds are in for a real treat, as Fillion is joined by some other space-faring actors, including Firefly co-stars Adam Baldwin and Alan Tudyk as Dutch and Mickey, respectively. And there's everyone's favorite Cylon, Tricia Helfer as Dare. The Joystiq staff is fairly certain that Romeo is played by Nolan "Nathan Drake" North, too.
Aside from nerding out over all that voice talent, we also noticed the video shows off a lot of neat stuff about the actual game. You should watch it. Right now would be good.