Did you update your Kernel? I had this problem you describe when I updated from Kernel 3.5 to 3.6.
Then I booted with Kernel 3.5 again (xdm working!) and then installed graphics driver for my NVidia for Kernel 3.6.
Maybe this will help you also...
I'm running SL X with Kernel 3.6 on an Intel Core I5 with NVidia GTM320 now.
This morning, I've made a big update including KDE 4.9.3.
As all I see, system is working include xdm (kdm)...
-Linuxfluesterer (I love KDE ...)
TTIP is NOT YET stopped. Especially consider NHS and ISDS topics. Talk to your local politician to open eyes for danger to democracy because of TTIP