A friend of mine is about to get married. He and his future wife don't want to spend a lot of money on the wedding invitations. As an (even cooler) alternative, he wants to setup a website about their wedding, not weddings in general but about this particular wedding. To pay for the hosting costs, he plans to put AdSense on it. Now we all know that weddings are a very hot AdSense topic so he's probably going to get a few good clicks.
He intends to spread the word about the site by email to all their friends and relatives. Would you say that this is against the TOS or might be considered click fraught? He wouldn't mention the ads in either the emails or on the site itself. But nevertheless the whole thing is quite risky, wouldn't you say?
If your site gets more then 100 unique hits a day then go for it. But dont put adsense on your site yet. You might one day get a high referral rate if a lot of people sudenly link to you because you posted something "awesome" :p cause then problems might arise with the hosting. Adsense could pay that off then ;)