Some guys with a posted video on YouTube appears to have found a way to glitch achievements in Gears of War 2.
The procedure is quite simple, actually, although you will need a profile that already have unlocked the achievements you wish to attain. Apprarently this is an Xbox glitch, meaning it will work for every game, although I'm not too sure about that. The steps are described in the video's details on the YouTube page.
This, of course, raises the morality question whether or not this is an honourable way to unlock achievements. Achievement glitches are one of the easiest ways for gamescore freaks to quickly increase their scores, but it sure does takes away some of the pleasure of getting the achievements.
I actually find myself in a hypocritical position regarding this subject, because I've personally used glitches to unlock certain achievements in Guitar Hero III, while I frown upon using this glitch to unlock Seriously 2.0. But then again, to each his own.