I definitely heard Nathan Fillion, Alec Baldwin, & Alan Tyduk of Firefly fame in the trailer. All three also had cameos in the first mission of Halo 3, Nathan Fillion's character even receiving the honorary surname of Reynolds to coincide with Malcom Reynolds, whom he played in Firefly. Alec Baldwin & Alan Tyduk randomly were heard throughout the rest of the campaign as Marines in your squad.
Confirms on them definitely being fully featured in ODST would be stellar. The preexisting chemistry between these actors is already self-evident in the trailer, & will make for a doubly-engaging storyline.