For a little while now Google has been offering a payment for referrals of an IE user that result in a "Firefox with Google Toolbar" installation. (that links to information about the program, not a link that would make me money).
<?php if (strpos($_SERVER['HTTP_USER_AGENT'],'MSIE')) : ?> --put your Internet Explorer specific advertising here-- <php else: ?> --either skip this whole block or put your non IE advertising here-- <?php endif; ?>
For what it's worth, I've noticed this problem on lots of sites...but got inspired to write about it when I was reading Alexander Kjerulf's Chief Happiness Officer blog. He's written some great stuff. It's only fitting he has a high performance (and high converting) site to go along with it.