Who Sees Ads by Ozh, is an advanced ad management plugin that lets you decide who will see your ads, for instance Adsense, depending on user defined conditions. You can manage ads in your templates (eg sidebar.php) or within posts and pages.
The plugin allows for easily creating display rules with a very advanced user interface.
Demo at: http://planetozh.com/download/plugins/who-sees-ads-demo/
Plugin homepage, config info and download: http://planetozh.com/blog/my-projects/wordpress-plugin-who-sees-ads-control-adsense-display/
Article and config info at : http://weblogtoolscollection.com/archives/2010/05/24/how-to-show-ads-to-only-search-engine-visitors-using-who-sees-ads/