There are plugins out there that attempt to do this. https://github.com/zookatron/sublime-collaboration is one, and another that seems a little more mature and usable is https://github.com/Floobits/sublime-text-2-plugin. But for me the biggest obstacle to this being actually usable is that the sublime text API doesn't allow making non-undoable changes to a buffer. This means that e.g. if you're making a change, then someone else makes a change, and then you press undo, you will undo their change instead of yours. If anyone is interested, I've made a separate post to request this feature at http://sublimetext.userecho.com/topic/158986-/.
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