had same problem as neophitezer1 with same (vt8375) chipset graphics.
in the device section, uncomment the use bios option and the crtonly option
that will limit you to 800x600 resolution, but when you quit xwindows, you will get back to the text terminal instead of hanging the pc with a blank screen. not a brilliant solution, but better than nothing!

not sure this is a fix but the problem is that KDE has past it over to kdm but a program has not been killed . and it is using a tty I find this is happening with the new firefox and the xulrunner set up with slackware 12.2. But that is the main reason a process is still running and it is a drag. I usually type control alt back because the kdm has crashed at that point. and now I can start to type. any way you look at it KDM has crashed do to a procces that has not been killed. Another mail program was causing this to only in my kde OS's so I guess kde is not removing the tmp sockets because of that.

i've tried ctrl-alt-backspace and it doesn't work, doesn't do a thing. I can't even switch to alternate terminal tty's with alt f1-f6
It seems as if the graphics chip is not being reset into the correct text mode on exit. I don't have this problem with nvidia graphics chipsets and slack 12.2 for example. It may be related to how the s3 chipset changes the resources it uses, but as generic vesa or vga driver gives the same problem it may be a more recent bug fix in the X drivers in general which break the s3 implimentation.
I didn't have the same problem with slack 10, although slack 11 I seem to remember gave a few problems, and i had to disable dri and glx to get it to work properly for s3 chipsets. As dri is built into the later versions that could be the problem.

I have this problem with the nvidia drivers and the nvidiafb frame buffers I will work on it from a xorg position and see what I find.
well I did a double check and it is Firefox update when this started happening. I can run my other program and no problems loging in and out. but the latest foxfire is not shutting down and it is hanging up kde. I am going to reinstall it and see what is happening. slack 12.0 I had this problem with noatun like yours i had no response and i just had to hit ctrl alt delete a couple times and wait for the reboot.

the program that was hanging up was foxfire. I deleted my /home/myuser/.mozilla/firefox that holds all my cache and ini and it stopped. Like I said this problem happend befor with other slackware and it was a program not shutting down.