Hi Chris, hope this video helps you: http://people.redhat.com/kverlaen/DomainSpecificWorkItem.swf. It's an older video but still really good...just make sure the file extension is .wid because this has changed since. If you are working with a maven project (not a drools project) it gets a little tricky, but you just have to make sure that the folders with your configurations are actually set as source folders in the project settings.
If you are using the new BPMN2 Eclispe editor I really do not know if this feature is supported there. The web designer fully supports workitem definitions saved in Guvnor, for this check out these videos: http://vimeo.com/24288229 , http://vimeo.com/26126678.
Hope this helps.