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So what about saying something like "Please check out the offers from our advertisers" (right under the banner). Or "Support this site by visiting our advertisers".
Legal or not?
If you are asking or suggesting people click on your ads I think your days will be numbered. Google wants the ad money and so do we, but I believe even more of a concern to them is keeping their money making adwords program in tact.
Once they feel there advertising money is in jeopardy because of poor roi and complaints from their advertisers I am sure they would cancel any account they felt was not properly serving adsense.
It certainly is not worth jeopardizing the program to make a few extra bucks that can be taken away as easily as it was given.
On the other hand, if some do try to entice the visitor to click and they are removed, it will make it more profitable for those that run the ads the way it is intended.