See the source code on this page:
http://www.bringdownie6.com/
Specifically,
Code:
<!--[if lt IE 7]>
<div id="ie6Warning">
<h2>Time to upgrade your browser</h2>
<p>If you're reading this, you're surfing using Internet Explorer 6, an eight-year-old browser that cannot cope with the demands of the modern internet. For the best web experience, we strongly recommend upgrading to <a href="http://www.getfirefox.com/">Firefox</a>, <a href="http://www.opera.com/">Opera</a>, <a href="http://www.apple.com/safari/">Safari</a>, <a href="http://www.google.com/chrome">Google Chrome</a>, or a more recent version of <a href="http://www.microsoft.com/windows/downloads/ie/getitnow.mspx">Internet Explorer</a>.</p>
</div>
<![endif]-->[FONT=verdana,geneva,lucida,'lucida grande',arial,helvetica,sans-serif]
[/font]