After some more hacking we have most of the functionality ready for real collaborative editing in gedit. Here is me and Armin collaborating (me with gedit and Armin with gobby, sorry, interface in Dutch):
In contrast to my previous post, this version now properly handles its sessions, does error handling etc. The only thing that is really missing right now is undo/redo, which is tightly coupled to GtkSourceView at the moment in gedit. We would need to make the undo handling less specific to use infinote undo/redo for shared documents.
When all of this more or less works, the next step will be to use telepathy tubes to allow sharing documents directly with one or more of your contacts. The idea is that you can offer a collaboration session to your contacts. gedit will then internally run an infinote daemon, offering a virtual document store made up of the open documents in the gedit window. All the necessary components are there, so it should be fairly straightforward to do.