Cannot use my Monitor with DisplayPort on 12.10: failed to train DP

Hi folks!

I'm having a bit of a strange problem here: I'm running Ubuntu on my MacBookAir5,1 and the only thing (!) thats causing troubles is the display:

If I connect to a Dell U2711 display via a DisplayPort cable, the internal and external monitor turn black and the whole system seems to hang. (I can't even switch back to the consoles). Using the same setup with a DisplayPort -> DVI-D adapter, both displays work fine. (Except that the monitor goes up to 1080p resolution then). I tried the standard 12.10 drivers and I've already tried the xorg-edgers ppa - both with the same effect.

Is there anyone around probably using the same setup (apple hardware) or some already reported issues I should have a look at?

Cheers,
 Sebastian