Well i see this will only bring minor changes tbh...
most people don't sign into their google account when they search for a keyword. It's actually different from FB where you are forced to sign in when you want to use it.People use FB on daily basis to check other status, post, pics, and video, hence why they click facebook like button easily...
non internet savvy users don't use google every day, even if they do so, they only search for 2-3 keywords then forget it. Most people who use Google for more serious keywords (not just funny stuffs) usually won't bother to go back and click "+1". People in FB usually click "like" button when they feel "agree" with someone's opinion or they laugh when someone post a funny status/pic/video. It's not the same with Google. When you want to know about specific medication, you will open 3-4 websites at once to confirm each other statement. You won't bother to click +1 to any of this website just because "now I know everything about this medicine". Usually people click "like" button when they agree, not when they get a new knowledge. Something that I believe why people use Google the most (because they want to know, they want to search)
It's like someone make a quote in Facebook saying "Great minds discuss ideas. Average minds discuss events. Small minds discuss people"
you agree with your friend and you click like button
but in Google, when you see a web page "It was roosevelt who made the quote - great minds discuss ideas, bla bla bla", would you click +1 button? I dont think most people will.
And I dont think you can compare this to adsense. Adsense is related to adwords. When an advertiser feels he get cheated by a pub, he will complain to Google and this is something Google want to avoid the most. That's why they are monitoring it closely. (they even have a team specialized for adsense, where they don't have such team for monitoring +1 thingey i guess)
and also, i think this will be gamed and tweaked by PTC people. There are tons of people out there who would be happy to make cents by just signing up, reading, and clicking websites. Imagine when a lot of SEOs could pay them a lot more for just signing up, reading, being active, and clicking the +1 button...(and of course stay hidden under radar, outside the usual PTC iframing)