Here it is in action (you'll need IE or some other crappy browser to see it):
[message about the user's inferior browser here]
...And here's the code:
<!-- the style should go in your head -->
<style type="text/css">
#crappy-browser
{
display: block;
}
div[id=crappy-browser]
{
display: none;
}
</style>
<div id="crappy-browser">[message about the user's inferior browser here]</div>
(from
MozillaZine)