Well, my problem is described in the topic: KDM starts normally, but if I enter correct password and hit 'enter', it shows black screen (for a second) and then restores login screen again.
The last two lines in kdm.log are following:
Also, I don't know if it's related, but I tried to install ATI Catalyst before it happened.
error setting MTRR (base = 0xd0000000, size = 0x10000000, type = 1) Inappropriate ioctl for device (25)
ddxSigGiveUp: Closing log
Lucid Lynx, 2.6.32-25-generic.
Thanks in advance,