At least on my Xbox 360 via Karaoke Revolution presents American Idol Encore. I played the short tournament (8 songs) and won, and then was mystified and annoyed when i didn’t unlock the American Idol achievement. Maj had confided that he had the same thing happen and we wondered if it was broken. But NO… you have to play the FULL tournament which is 18 freakin songs, plus suffering through Randy calling you "dawg", Paula emoting, and Simon being all…. Simony. (For the record I have never watched the show. Not. Even. Once.)
But finally, I was most triumphant and unlocked that bad boy for a whopping 50 Gamerscore!