Bug#472318: Looks like a bug in my PAM configuration:

[John Hughes]

Mar 23 14:18:13 carbon gdm[882]: PAM _pam_load_conf_file: unable to open 
/etc/pam.d/common-pamkeyring
Mar 23 14:18:13 carbon gdm[882]: PAM unable to dlopen(<*unknown module 
path*>)
Mar 23 14:18:13 carbon gdm[882]: PAM [error: <*unknown module path*>: 
cannot open shared object file: No such file or directory]
Mar 23 14:18:13 carbon gdm[882]: PAM adding faulty module: <*unknown 
module path*>
Mar 23 14:18:13 carbon gdm[882]: PAM error loading (null)
Mar 23 14:18:13 carbon gdm[882]: PAM _pam_init_handlers: error reading 
/etc/pam.d/gdm
Mar 23 14:18:13 carbon gdm[882]: PAM _pam_init_handlers: [Critical error 
- immediate abort]
Mar 23 14:18:13 carbon gdm[882]: PAM error reading PAM configuration file
Mar 23 14:18:13 carbon gdm[882]: PAM pam_start: failed to initialize 
handlers
Mar 23 14:18:13 carbon gdm[882]: gdm_verify_user Authentication failed: 
1328: pamerr=26

Looks like some leftover junk in my pam.d/gdm file.

PEBKAC, sorry.