Nathan Fillion

Go to 37 seconds in and listen to the Marine that says "Let’s mount up, get the hell out of these caves." Sounds just like him.

And he speaks again, at 2:17; "Best thing now, get some distance between us and the base. Brutes are going to be looking for survivors."

For those that don’t know, Nathan Fillion plays Mal in Firefly/Serenity.

My sources tell me Halo 3 should have quite a few surprises in store for Firefly fans.