I found two solutions on my first Google search.
Code:
<!--[if lte IE 6]>
<style type="text/css" media="screen">
div#mainContainer {
width:expression(((document.compatMode && document.compatMode=='CSS1Compat') ? document.documentElement.clientWidth : document.body.clientWidth) < 1000 ? "1000px" : "auto");
}
</style>
<![endif]-->
Code:
#div {
min-width:100px;
<!--[if lte IE 6]>
width:100px;
<![endif]-->
}
Naturally none of these items will replicate the min-width property as IE6 doesn't support it but it's about as good as you'll get.