I'm trying to finish up a fresh install of gentoo, and I've run into
the following problem:  when trying to start xdm, the screen turns
black, flickers once, and the system freezes.  According to the xdm
log (attached) I get errors regarding AGP, sometimes xf86_EINVAL and
sometimes xf86_ENODEV.  I tried setting the permissions mask in my
udev rules to 777, and now xdm starts up, but freezes on exit. 
Running startx both as root and as a normal user works fine, but xdm
has problems.

I've had this working before on the same hardware, using the same
config.  A different kernel; the working kernel was 2.6.11 whereas
this is 2.6.12, but all the relevant kernel options (framebuffer, drm,
etc) are the same.  So far the forums and googling have yielded
unhelpful results, except for the suggestion of changing the
permissions, so I'm assuming I've done something boneheaded that
perhaps the list will catch.  Thanks in advance.