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If you compare your logs with what you are actually being charged for you may notice that you're not getting billed for some clicks.
Overture do have some sort of system in place that seems to catch a lot of fradulent clicks.
Regardless of the PPC provider, I recommend you keep a close eye on your server logs & use tracking software that logs the referrer source & visitor IP addresses since you won't be able to get a refund if you're not looking out for fraudulent activity or have the proof to back up your claims.
Overture does a good thing - they allow you to identify their paid clicks, regardless of partner, as long as you tell Overture to auto-append the "ovraw=" parameter to your landing page URL. With Google, you can get the same thing but you have to do it manually. Add a similar parameter to the target URL you specify when you set up Google AdWords. The Edit Keywords box is the easiest place to do this on Google.
I swear, if only these secondary guys get their act together and catch (not bill) for the publishers that generate bad clicks, they'd get more advertising $'s.
I should add that hardly any of Overture's clickthroughs actually had Overture as referrer. And I learned a lot about the identity of search and content partners - quite a few minor players, but nobody I'd object to except maybe one or two (Amazon for example is not at all appropriate for us, no matter how you argue it).
The only way I can see to watch for this is to micro manage you reports on the site for a short time after you change - then see if you are still getting visits at the old (and, of course) higher bid.