Hydra is a very nice implementation of this idea - you share your document to the net and people can edit it at the same time as you (you get to choose who). It uses Rendezvous and Addressbook, so you get user icons like iChat.
Jem wrote something very similar nearly 10 years ago before we dragged him into Newton development. His ran over LocalTalk, but it did allow two people to modify the same file at once.
NB Hydra is OS X only.