Comment 9 for bug 1165965

The same problem (external interface DP1 showing as disconnected, sometimes showing the resolutions) happens to me on my Macbook Air 4,2 since already Ubuntu 12.10. This affects also derivatives like Mint and also other distributions like Archlinux. From this I would conclude that it is a general problem with the kernel and/or Intel Xorg driver.

The problems depend a little bit on the external monitor that I am using: A normal 19'' LCD with VGA connector works sometimes (after rebooting, suspend/resume, disconnecting/reconnecting). A no-name LCD TV (also VGA connector) doesn't work even with trying all of the things that worked for the 19''. The problems seemed to be gotten worse with Ubuntu 13.04 in comparison with 12.10. With 12.10 I could also get the TV to work after playing around a little bit.
As I am using the Air to give talks, this is a major deal-breaker (as also projectors don't work anymore) and the only reason why I still have to boot Mac OS X (there everything works fine).

If i can help with logs or testing something, feel free to tell me.

Another thing: What I can do is force the monitor to some resolution. As Michael mentioned, the monitor is then switched on, but as xrandr still shows the monitor as disconnected, one can't really use it.