I found a thread at DigitalPoint Forums that is an offer to anyone who wants to rent out one of his AdSense units.
So he has a forum, and he wants to rent out on a monthly basis, one of his ad units. In this case, he wants to rent out the ad unit at the bottom.
Should you do this? Should you rent out ad spots? Should you pay someone for an AdSense unit spot?
I don't think so. I think it is a lot like those YPN partnerships. But in this case, you manage the content on the site. So you as a publisher won't be affected that much. But if you rent out the spot, you do not have control over the content - so there is room to worry.
Forum discussion at DigitalPoint Forums.