Yeah, I once read about a web company that lost a month's worth of ad revenue ($10,000 or $100,000, not sure) because there were people that clicked their ads repeatedly, and Google shut 'em off. Not sure how much that would be though. But just remember that each time you click an ad, not only do we get a few cents, but another person has to pay that same amount.
Not sure how Google's protecting that though - but I'm kinda sure there's prevention measures that'll prevent clicks from being counted if they suspect they're not done 'seriously'. So, don't 'spamclick' 'em. I would say 'Feel free to click on ads' before that sentence, but I'm not officially allowed to ask or request or make people click ads.