A commenter asks: "Do you think Google will ever change to a pay per acquisition model?"
One of my basic laws for internet marketing is that nothing ever works well for more than 3 years. By the end of 2006, PPC won't be effective for a large number of online sellers. Google will have to adapt, and they will. It doesn't mean they'll stop the CPC ads, but they will somehow offer a CPA model. If they wanted to, they could do it right now, better than anyone else...
Also see my earlier post on how Google has a marketplace - much like a stock market, where they control all the rules, and that will be to their advantage when they need to adapt.