Are you logging in as root? If so, it will fail because the default for kdm is to block root logins. That behavior can be fixed by editing /usr/kde/4.0/share/config/kdm/kdmrc (I *think* that's where they hide that file). However, you'll have to enter your Gentoo environment through chroot in order to edit it.
Blessed be!
Pappy
This space left intentionally blank, except for these ASCII symbols.
I figured it out. I had copied a version of "xorg.conf" from a user and their keyboard was set to "fr" instead of "us", so I was typing the wrong characters.