I once had experience of writing "collaborative" text in LaTeX by using subversion -- a software versioning and a revision control system for program-developers. That was really cool! Everytime you have the most up to date version. All the "collisions" are dealt with automatically e.t.c.

The problem is -- all your collaborators has to be familiar with this software and the concepts. That's why I had such experience only once.

But I still use it for my own projects -- I setted up repository on my usb dongle, so I do not depend on the computer I use...