2.2.3 is the version im using, I used to be able to get voltage unlock on my old 460, only time I have seen this myself (apart form now) was when my mate got his 570 twin frozr and after a year it still doesn't allow voltage unlock.
Been tinkering about with the unofficial method this evening and no joy, may just have to use the asus software.
Which card have you got?
Maybe its a known issue with afterburner?