I received an email today, informing me that Google would be adding the +1 button to Google AdSense ads. Google's Inside AdSense blog explains the details further. You will be able to see that your friend has +1'd the ad, along with any of your other contacts. The recommendations will be used to create more relevance for you, as the user.
Honestly, though...I'm not sure how I feel about this. I prefer ads to be unobtrusive, and this seems too much like turning my friends into advertisers.