Google is moving to reduce accidental clicks.
This post post explains it well http://blogoscoped.com/archive/2007-11-13-n42.html but you can read it at Google Adsense Blog. This could mean decrease in revenue for some. Despite that I think it is a fair change as it gives the advertisers better value for their money. I am not sure why it was not like this from before.
Happy advertisers are likely to keep on advertising. This quote from the the article sums it up. It says, "By reducing accidental clicks, we hope to increase advertiser campaign value and satisfaction, encouraging additional spend and facilitating higher monetization for all publishers."
