Typically you should be wary of using "dist-upgrade" during the development release, if the archive is in an inconsistent state it will remove things and this will happen. This always isn't easy as you might have upgraded part of the system and the bits that they depend on might not be build yet.
Normally when this occurs packages to be "held back", but a dist-upgrade overrides that and just installs the newest things in the archive, whether that breaks your computer or not.
The simplest way you can recover is find out what you're missing and reinstall it, I would guess in your case you upgraded either X, nvidia, and/or your kernel and your mirror wasn't caught up or the right things weren't built yet. This is why I always upgrade with "apt-get upgrade" or the update-manager and just let things sit held back for a while.
What I do is do
apt-get upgrade, then when certain things are held back I apt-get install them one at a time. So for example kernel packages will be held back and I manually install them. If something wants to remove something that looks important I keep it held back. It's not too time consuming and ensures that something important isn't removed.