The easter egg of Firefly's involvement in the Halo 'verse is a bit deeper than what's been mentioned here, even.
Nathan Fillion's character is definitely referred to as "Sergeant Reynolds" several times in Sierra 117; he's the guy Johnson is talking to on the comm initially when you move out, & he's also the guy you rescue from the Brute (if you're quick) a few minutes in.
Alan Tyduk is the guy taking Chief's vitals in the opening sequence, & as already mentioned has quite a few humorous lines as a "generic" Marine throughout the campaign.
Alec Baldwin gets some of the funnier tough guy lines throughout as well, & it appears as if all three actors are playing characters not far from their Firefly roots. Baldwin's got his "strong, silent type" line in the ODST trailer, & Fillion's got just enough of his Firefly drawl present in "you know the music, now's the time to dance" to remind you of Mal.
Anyways. The opening of Halo 3 feels so good because of the familiarity in the dialogue to the Firefly 'verse; so glad to have that easter egg furthered in ODST & (potentially?) Reach.