HTML5’s Video tag already works in IE
Just add these two lines to the top of your page:
<script src="http://visitmix.com/dlr/dlr.js" type="text/javascript"></script>
<script src="http://visitmix.com/dlr/gestaltmedia.js" type="text/javascript"></script>
<script src="http://visitmix.com/dlr/gestaltmedia.js" type="text/javascript"></script>
Of course, this is a hack workaround that uses Silverlight to emulate the Video object, but going from “doesn’t work at all” to “make users install a plugin, once” is a pretty good deal. Here’s the original post describing it.