It's a new program by google, to get adwords users to target advertise for your site specifically. It means that they will pay for views, I think, but I have not fully researched it. I thought it would not show until December.
Log into your AdSense account and under My Account you can change it (like add your logo) or opt-out completely.
This has probably been discussed, but I wonder why it's opt-out instead of opt-in. Also, anyone who missed the official AdSense blog entry about this: http://adsense.blogspot.com/2005/11/gain-more-advertisers-right-from-your.html
Onsite Advertiser Sign-up: the short version: "And starting within the next two weeks, advertisers will be able to bid for placement on your site right from your web pages", but yes, opt-in by default. You don't get paid per referral, so I can't see how you would get banned by clicking on that link.
Odd.....just a few days ago wasn't this an opt-in feature? I think this is due to competition with programs like Adbrite? They make WAAAAAAAAAAAAAAAY more money per week than Google Adsense could make you (with the same amount of effort imo)