Curtousy of THC, enjoy
Note: I don’t think this a leak since it was at the Halo panel at SDCC. If it is leak me know and I’ll remove the link.
This one ain’t the trailer version either. It’s actually gameplay from a mission.
http://www.youtube.com/watch?v=KvAp37HOx44&feature=g-all-u
Seeing some of the footage is from SDCC at the Halo panel there, I don’t know what is linked content and what isn’t.
Also, the gameplay begins at around 2 minutes.