With PPC engines (especially Overture, since I'm vastly more familiar with their inner workings), I really don't see why you'd want to intermediate your traffic flow with an FPA.
We all know surfers have short attention spans -- if they see a SERP and click on a link, they're saying they want that content, and they want it now. They don't want to be bothered with more ads -- basically, if you give them an excuse to get out of your sponsor's purchase pipeline, they'll happy do it and go somewhere/anywhere else. Hopefully you'll at least catch them with a console, but that's another story...
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