mildly relevant: a web-based, collaborative latex editor.

interesting: writelatex is a latex version of etherpad, a collaborative writing and editing tool.

i would imaging that coupling dropbox with your favorite $\LaTeX$ program would accomplish something similar but without the option of editing the same document simultaneously.

(to me, though, the only instance where this could be really useful would be if several authors are composing the introduction of a paper together.)