excanvas is pretty slow, I would personally prefer to just drop support for IE <9 than to go through the trouble of making sure it works with excanvas.
Also, canvas is just one of the html5 features it uses (there's e.g. also audio).
You could also add Google Chrome Frame to the page instead of outright dropping IE <9 support. At least it gives them another option, especially while IE9 is still in beta :)
How much of it works with excanvas in IE <9? http://code.google.com/p/explorercanvas/wiki/Instructions