This is only going to stuff 1:10, however, it really isn't a 10% CTR because there will still not be any impressions from the forum. If 100 visitors pass by your code, only 10 will be cookied, but there will not have been any ad impressions, thus in reality, it would still be 100% CTR. Correct? Or am I missing something here?

The way I see it, you would need to rotate the impression pixel the other 9 times, that would have the desired effect of lowering CTR.

Liam