Odd problem with PAM and KDM

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

I have a laptop which has been running Debian for around 2 years now, with
a user ID for me which works just fine.  I can log on to a line mode console
or use KDM to log on to KDE - no problems.  The machine is running Sid,
and is pretty well up to date.

Recently I added a user ID for my son so that he could use this machine
for some school work.  He claims he did nothing out of the ordinary, but
after a history lesson he stopped being able to log into KDE.  

When I got a look at the machine there were no obvious errors, and he
could still log on to a command line console and issue startx to get into
KDE, but just could not get into KDE from KDM.

KDM reports immediately that there is a logon failure, and there is a 
log message in /var/log/syskog which reads:-

kdm(pam_unix)[843]: authentication failure; logname= uid=0 euid=0
  tty=:0 ruser= rhost= user=michael

It then says, just as it would for a normal login:-

logon(pam_unix)[686]: session opened for user michael by LOGIN(uid=0)

and that it all.

Anyone got any ideas as to why this might be or how to debug it?

Thanks

David