AdSense have just announced in emails to publishers that they have decided to remove the ‘Onsite Advertiser Sign-Up’ feature that once appeared occasionally on some ad units (the link used to say ‘Advertise on this site’).
According to them, the removal of the link is due to it’s poor performance and will soon be completely gone.
Personally I see this as a good move by them. I can’t imagine many people just clicking on that ad, signing up as an Adwords user and going from there. It’s the same as a person just clicking on ad to buy a new mobile phone (the ad was on a blog let’s say) and going through to buy it that moment. It’s very unlikely, and usually the clicker has to think about it, and then will by from the cheapest option. So that link probably didn’t get too many new Adwords sign-ups for Google.
Over at Adsense they’re really trying to revamp the system. Just lately they’ve even changed ads from being clickable all over, to only clickable on the title and URL. You might want to read this for more info on that change.