I believe that my hardy install had the kubuntu-kde4 packages installed as well, but I wasn't using them and switched back to the kdm login screen.

I upgrade, it apparently goes smoothly up until i reboot then I get this error message:

grep: /usr/lib/kde4/etc/kde4/kdm/kdmrc: No such file or directory
Not starting K display manager (kdm-kde4); it is not the default display manager

which is obvious since there's just kde 4 now and it's just called kdm not kdm-kde4

I was about to start poking about at the init script but found this thread which is pretty much the same problem and the described fix makes perfect sense, uninstall kdm-kde4 and reinstall kdm with these commands.

sudo aptitude purge kdm-kde4
sudo aptitude reinstall install kdm
sudo depmod -a
dpkg-reconfigure kdm

and then I reboot and I don't get the error message anymore but kdm still hasn't come up