I have seen it in a couple of sites.
Anybody has seen it ?
The adsense stays on its place as you scroll down the page
its a nice effect and it could be getting more clicks then usual
What do you think ?

Is it legit, or would one be banned for that ?

I looked and I didn't see any rule prohibiting it so I say go for it. If you are Leary I would email Google and ask them.

Thanks for the looking into it and for the offer to email google, but I was mostly interested in everyone's personal opinion on effectiveness of ads and or legality.

Its really like shoving an ad in someones face. I think I would steer clear of that. If it is not banned it should be.
Its just like a popup... I really don't think visitors will like it either.

I think it's fine ... like if you had it in a right column and that column didn't scroll, to give the illusion of frames.

Nothing in google's tos says it's not allowed since you're not directly editing any of the actual adsense code.

I am agreed with mitz. It is like a popup. Though Google's Tos doesn't prohibit you to do that directly It is not a good idea to provoke user to show a specific Ad Space. In my sense it will get higher CTR which is also very dengerous to get google's attention. You should better ask the Google Experts before applying the concept.