Missing a few from the first borderlands on xbox inlcuding:
Speedy McSpeederton- Race in under 31 seconds
Can't We Get BEYOND Thunderdome?- Win an arena match
There's No "I" In "Team"- Complete 15 missions in co-op
And They'll Tell Two Friends- Play in a co-op game with either an employee of Gearbox or someone who has this achievement
And the secret achievement for killing a boss in co op.