In the last few weeks we have been adding more and more users to the new GNOME desktop server and hit a tuning issue where it would stop allowing people right at 80 concurrent. My first inclination was that it was a tuning issue with GDM and I had a few IRC conversations with the ever friendly Halfline. I found some error messages in /var/log/messages and after Googling was able to find the cause: Dbus. There appears to be a bug in the code that counts the number of concurrent processes from the same user account. My reading indicates it's supposed to be 256, but others have reported it stopping far short.
So someone working on Hardy found a fix, which I copied and it works on OpenSuse 11.4 as well. You add a system-local.conf file to /etc/dbus-1 with the following XML parameter. After this change was made, we now have hit 86 users with excellent results. I'll blog in the coming days about the user loads, so far we're very pleased.