The ebuild for openssh-5.0_p1-r1 was NOT based on 4.7_p1-r20, which installs a new version of /etc/pam.d/sshd that relies on the new pambase package. Flameeyes confirmed that the ebuild needs to be corrected.
it was done originally because things werent KEYWORDed ... since they are now, it can be updated (use a real -r# this time, not jumping to -r20 :P)
Fixed in 5.0-r2.