Aren't there more ways to detect if it is the same person? How about their writing style, typos, punctuation, pages visited, screen res, plus.. can't you block those proxy sites on a list? I know it is not nice to block all proxies.. but I'm just asking, because I am not sure how this works.
I am a member of a community that shares adsense and obviously you're only supposed to have one adsense ID and you don't get in the community without one, so at least it seems to be a clean place with value and honesty. Hopefully people won't sign up with others' account numbers, tsk tsk!