Setting up a used projector as a third monitor is a success! Shown here running XBMC on OS X. Look how tiny the 27″ and 23″ monitors look. :)
The projector is connected via VGA and the 23″ monitor via USB2-DVI adapter. The latency on the latter isn’t fantastic, so it wasn’t appropriate for the projector since I mainly use it for movies, video conferencing, music visualization, gaming and other graphics intensive applications. It’s entirely fine for the 23″ at 1920×1080 for what I use it for: email, IM, IRC and Skype (text).
All in all, I’m very happy with the setup. Now, all I need is to build a hushbox to quiet down the fans an mount it to the ceiling.