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The nice thing with this method, is that you can have a text-based replacement version of the ad in a <div> that SEs actually can see/index. So if you put your link in this text you will get an inbound link, because the SEs never see the flash-ad. Visitors who has the Flash Player, will get the flash-banner-ad instead of the text-ad.
Look up SWFObject to get more info on the actual implementation.