xorg-x11 used hard-coded paths in it's pam.d modules (/lib/security/module.so). This causes problem on multilib systems and on gentoo/fbsd as the pam modules are not stored there (works atm on amd64 due to out hacky lib -> lib64 symlink but this will be removed at some point). The full path is not needed here as pam will locate the module in it's standard path (set at pam compile time) if the module is listed without. In short please just sed -i 's:/lib/security/::' files/{xserver,xdm}.pamd and everyone should be happy.
Does this bring in any dependency on a minimal version of pam for this to work?
Fixed in 6.8.2-r2, 6.8.99.8. Leaving open, pending a response to comment #1.
No, all pam versions in portage responds in the same way to modules without fullpath.
Thanks for the quick response =)