That would be pretty awesome actually. All of the noted features, but particularly syntax highlighting and some sort of versioning. Key use case would be "multiplayer programming," rather than just one person driving while the other person stares at the screen over their shoulder.
TeX and Markdown would be good, but they strike me as different enough uses that maybe they'd be a separate app -- with TeX or any other markup language, you'd want to be able to plug it into a publication system, or at least generate nice output (whether DVI, PDF, XML, HTML). Lot of users there as well, but they might be different users.
