These are the new achievements added via Live, they are all multiplayer. Worth 75 gamer points total.

Achievement	How to unlock
Assassin (Multiplayer)	Find and kill an opponent who has the Assassin Achievement.
Crack Shot (Multiplayer)	Kill 10 players with gunshots and at least 5 gamertags in the room without reloading or dying.
Explorer (Multiplayer)	Win 5 Team or Solo matches on each original MP map with at least 5 different gamertags in the room.
Team Player (Multiplayer)	Win 30 co-op matches with at least 6 gamertags in the room.
Victor (Multiplayer)	Win a public player match in all original game types with at least 5 different gamertags in the room.