I've been googling for some time and didn't find any solution.... Here goes my problem:
At the login dialog in KDE when I choose a regular user, the dialog disappears, but nothing happens. As root everything goes just fine.
This is the .xsession-errors from the regular user's home, after trying to start kde (alex is the normal user):

xset: unable to open display ":0.0"
xsetroot: unable to open display ':0.0'
Error: "/tmp/kde-alex" is not a directory.
Error: Can not create directory "".
Error: "/tmp/ksocket-alex" is not a directory.
Error: Can not create directory "".
startkde: Starting up...
startkde: Running kpersonalizer...
SocketUNIXCreateListener: mkdir(/tmp/.ICE-unix) failed, errno = 13
MakeAllCOTSServerListeners: failed to create listener for local
Cannot establish any listening sockets
DCOPServer self-test failed.
kwin: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0
kpersonalizer: cannot connect to X server :0.0

I've tried xhost +localhost but it didn't work. Any ideas?
Thank you for your help.
PS: I'm running (or trying to run) Gentoo with xorg 6.8.0-r1, kde 3.3.0

Are you running the Xserver? Refer to gentoos manual on how to install KDE with Xserver. It's on thei rwebpage under post install instructions.

Try creating a new user and see if the new user works.