KDE wont start on unstable after upgrade

[David Goodenough <david.goodenough@btconnect.com>]

I recently upgraded a machine which had been installed as unstable but had
not been upgraded for a few weeks.  Everything has upgraded cleanly, and the
machine used to work quite satisfactorily.  Now when I boot the machine it
starts KDM quite happily, but then when I login using kdm it clears the screen
and then redisplays the kdm dialog.  Both the kdm dialog and background appear
to be normal and there are no errors in /var/log/XFree86.log.0.  This happens
whatever the user (there are 2 normal users + root on the machine).

Login on through one of the tty consoles works just fine for all the IDs, and
exactly the same behaviour happens if I logon at the console and then issue 
startx.

I can not find any error messages anywhere, anyone got any ideas?

David