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I was actually thinking along the lines of clarifing the fact that these are adverts, not links to other parts of the site, so as to prevent confusion on the part of the users. On the other hand, I'm not that bothered about it ;)
However, I was musing about where the line is drawn between "encouragement" and "clarification". For example, you cannot draw attention to the ads. But, what about on your "disabled" version of your site? Adding a sign simply saying "Ad" would certainly help users with bad eyesight, etc.
On the other hand, I rather that they did click on the ads, rather than stayed on my site, heartless b*****d that I am! :)