Comment # 1 on bug 962480 from Dominique Leuenberger

The problem is in /etc/pam.d/common-session:

session    optional    pam_gnome_keyring.so    auto_start
only_if=gdm,gdm-password,lxdm,lightdm

This list needs to be extended to also add mdm to the list of valid DMs here.

Can you please verify this locally for a test and report back?