This sounds like same or similar problem to mine, except I use xdm and openbox on debian sid. I'm searching through the xdm config files and man pages to figure out how to effect the described work-around but for xdm instead of kdm.
OK, looks like I was right about similarity. Found solution in debian bug report for nvidia driver.
Adding this line to my /etc/X11/xdm/xdm-config works around the problem for now:
Thanks to all who have input in this thread.