I started up firebug to debug it and I can see an error when the desginer tries to load a BPMN2 diagram when I have my custom Work Item Definition.
Here is what the firebug output looks like when the designer loads up fine (without any custom work item definitions)
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-820055-20877/450-198/Firebug+output+normal+loading.png
Here is what the firebug output looks like when the designer gets stuck in the "loading" screen (when I have a custom work item definition)
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-820055-20878/450-197/Firebug+output+error+loading.png