KDE workstation install. At kdm, select 'GNOME' as the session type, and log in. You are dumped back at kdm with no explanation ("well why did it offer me the choice then?"). It shouldn't offer session types that aren't installed.
Any suggestions on how to do this? We'd basically have to generate them on the fly while they're a static file. I don't see any real way to fix this (except for, of course, rewriting kdm)
How about %post sections in the RPMs for (e.g.) kdebase and gnome-core to fix up the session types list? Or an initscript to figure out what session types are available or something.
I'm going to hack KDM to understand the gdm session stuff for the next release.
scratch that, it would be far too much work.