Comment 43 for bug 38915

My debian/testing system worked fine. After installing kubuntu 7.04 I got the crashing shut down problem.

So similar to the solution for gdm-users, if you use kdm:
As stated here

http://gentoo-wiki.com/HOWTO_ATI_Drivers#System_freezes_after_logout_with_GDM_or_KDM

you've got to add the line

TerminateServer=true

in the [X-:*-Core] section of the kdmrc-File (/etc/kde3/kdm/kdmrc) - beware of the ':', there is a similar named section without it, too!
Seems to work for me, restarted my box now three times without problems!