The Torque Game engine has this for short time. One of the community members released a mod to the engine that allowed upto 32 players to work on a single game world at one time.
With the tree view setup in the editor something like this could be possible with afair amount of work. Really if you think about it, its nothing more then spawning objects into the game world and saving them to the map. In that case the basics are already there inside of the engine / editor. You would need to come up with a way to keep the artwork (models / materials / blueprints) syncing up with each other. (Like a svn / github / perforce style system in realtime.
Doing something like multiple people working on the same blueprint seems like it would be out of the question. But just simple populateing the game world with models wouldnt be out of the question. Terrain editing would be a different story.