Read the news much? You can add your retail game to Steam, but that's the only way to get the achievements.
I thought I did read the news ... I knew you can add UT3.exe as custom link, but does that unlock the achievements? Last I read, it doesn't unlock the download option for one, makes it doubtful the achievements are unlocked...
But I probably missed something there. Can you give me a link to bring me up to speed?