Here's a good tip I read quite a while ago, but forgot to pass on to you...

As more and more people switch to tabbed browsers like Firefox and Internet Explorer 7, a good place to put your ad blocks is right up at the top of your page, immediately under the area where the tabs are displayed.  The ads should be the first thing on the page, no space, nothing else above them -- this works for contextual ads like Google Adsense as well as more typical banner ads.

This seems to work because people focus on the tabs much more than they normally would when using a browser that does not include the tabs.  Give it a try!