Some of you might have run into this (SORRY!), but as agreed as a targer, packages that do support it, have now systemd enabled (most notably, that means they use systemd-login instead of ConsoleKit).
Unfortunately, gdm failed me on that one... and only comes up with a 'oh no, something has gone wrong screen'.
The log files were rather obvious, on what's going wrong though.
So, if you see this, please get the polkit packages from Base:System (current version 0.108) and install those. It has as well the systemd integration enabled.
Make sure to kill polkitd or restart your system, as otherwise you end up with an updated package, but an old polkitd in memory... no win :)
For the ones that were hit by this, sorry for that.