Google ad-sense program will help you to identify your most cruel concurrents
I believe the most web-masters knew this, for me it's like a game now: we've recently installed Google ad-sense frames to Milan-IN, and now it's my job to protect our partners/sponsors from the ads of their concurrents appearing on the pages of Milan-IN.
We must blacklist all 'undesired' ads brought by Google, that's brave to analyze the content of our pages and connecting the ads to them. And I am very glad that Google ad-sense programs support blacklisting!
Just for fun: my advice to those web-masters that still did not try Google ads would be to try it, at least to see who from your concurrents pay for the on-line advertisements :) You'll be able to build a great list of your concurrents, after few days of watching Google's ads proposals for your web side (I hope it's clear that it is always possible to place the page with ads outside of your web site client's visibility zone, to a place available only for your eyes).
I do not know if it's a violation of the Google ad-sense program license, I do not think so, but in any case we (Milan-IN) use Ad-sense not to just play with it, and the only thing that's missing is to 'protect' our Partners/ Sponsors!
The others are willing to do whatever they want with their web sites and with Google-ads, of course if it's not the policy violation!