I gave some thought to the click fraud problem tonight. While I agree with Bruce Schneier that pay-per-action is the right solution, a solution is needed now, not years later, and pay-per-view/click model needs to be shored up even if pay-per-action business gets traction.
This is the solution I arrived at using two glasses of red lube ;-):
seed each clicker with a cookie to identify the computer.
void clicks originating from machines without the cookie.
void clicks originating from machines exhibiting suspicious activities detected by offline analysis.
While this solution requires pay-per-click ad vendors (i.e. Google) to cut into their profit for now, I think it's worth implementing in the long run.