Processing in java is a really nice project and it would be really nice to be able to run processing visualization in the browser without having to run java applets, but the current lack of support for Internet Explorer (I do not know if it would even be possible to realize, BTW) is really preventing me to invest too much time in it. I have been using Raphael.js instead.
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 :)
