Is click fraud via Google “a fraction of one percent”?

Donna Bogatin has an interesting post on ZDNet entitled, Google and click fraud: As wholesome as milk?

While giving Google left-handed praise for “playing a good PR game in the blogosphere,” she questions “the double speak nature of Google's click fraud stance”. According to Google, its advertisers are charged for “valid clicks,” but not for “invalid clicks.” And what is Google's definition of an “invalid click”? “Any click Google doesn't charge for.”