IE is not supported by the designer. We're testing whether it can be used though using a Google Chrome frame inside IE. Check out http://community.jboss.org/thread/160888?tstart=0 (latest posts).
I tried GCF today and it worked for IE. I was able to see the designer both on IE6 and IE9, before they did not show up at all with errors, so it works the same as in Google Chrome. But as you said there is a bug in editor, the message box does not go away, but that is something else. I am happy that GCF worked .