Just found this on Zavvi and think it's a pretty good deal.
It got some good reviews:
IGN - http://uk.xbox360.ign.com/objects/852/852871.html
Gamespot - http://uk.gamespot.com/xbox360/action/halo3untitledodstgame/index.html?tag=result;title;0
Here's some info (taken from Play.com):
"First-person shooter 'Halo 3: ODST' is an epic standalone expansion for Halo 3 and is exclusive to the Xbox 360. Players take the role of Orbital Drop Shock Troopers (ODSTs), an elite human force during the events of Halo 2 and 3. After the devastating Covenant attack on the earth, players are dropped into the ruined city of New Mombasa and must explore the ruins in order to find clues and discover what happened to their fellow soldiers.
This new campaign gives a whole new perspective to the Halo universe and its new multiplayer maps ensure that the Xbox Live experience is as good as ever. "