Seemingly lost in the shuffle of a busy week, the new achievements, per Xbox.com:
• Undisputed: Complete Ahern's survival mission (50 Gamerscore)
• New Sheriff in Town: Take first place in any combat scenario on Pinnacle Station (50)
• Best of the Best: Take first place in 12 combat scenarios on Pinnacle Station (50)
This raises the overall Gamerscore for Mass Effect to 1,200. No word exactly when the DLC gets here.