I recently added a feature to the JavaGameNetworking (JGN) project to allow for graphical synchronization. Because I use jME exclusively, I have created a plugin for providing this functionality. It would not be that difficult to also provide that functionality for Xith or Java3D if there's interest. What this feature provides is the ability to register your graphical objects to send updates to the server or other clients and synchronization in the game space comes automatically.
Check out http://www.captiveimagination.com/forum
for further details.