I have yet another solution, and it's a really squirrelly one. I did look at the other solutions posted here, but they didn't help in my case.
I was also getting kicked back out to the kdm. So I wanted to see if this problem was affecting the whole system, or just KDE.
The first thing I did was reboot my whole system (PCLinuxOS), and try to login through kdm again. It failed again.
So, on a lark, I selected a GNOME session and successfully logged in. Everything seemed OK, but I don't want to use GNOME. (No time for religious wars right now, so I don't care to get into these preferences...) Then I logged out and back in again in GNOME, and then exited. I selected KDE again, and ... viola! KDE login worked again!
Looks like changing the session type somehow jogged kdm back into health. Bizarro, really.
One thing that perhaps might help explain some things ... the previous day, I got an email from my system warning me of publicly writable files in the /usr/share area. So I did a find and chmod to take just write privs off the suspect files for world, but not owner or group. For a while, I wondered if that might have somehow had something to do with it. Maybe Gnome resets these file perms when it comes up for the first time, or something like that? However, I don't notice any new write perms in those files after logging in. But maybe I have not looked at everything yet. Anyhoo ...
I hope someone else with a similar problem will google this thread and it will solve their crisis too.