Here is the samba log for one of the client computers (/var/log/samba/log.lpfrontdesk-2):

[2012/06/16 06:02:44.157953, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_S
erverAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth
request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 06:13:25.884889, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_S
erverAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth
request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 06:28:28.958987, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_S
erverAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth
request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 06:30:16.498471, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_S
erverAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 06:43:02.623006, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_ServerAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 06:51:49.072362, 0] rpc_server/netlogon/srv_netlog_nt.c:976(_netr_ServerAuthenticate3)
  _netr_ServerAuthenticate3: netlogon_creds_server_check failed. Rejecting auth request from client LPFRONTDESK-2 machine account LPFRONTDESK-2$
[2012/06/16 17:54:42.137405, 0] auth/pampass.c:797(smb_pam_accountcheck)
  smb_pam_accountcheck: PAM: Account Validation Failed - Rejecting User LPFRONTDESK-2$!
[2012/06/16 17:54:44.541723, 0] auth/pampass.c:797(smb_pam_accountcheck)
  smb_pam_accountcheck: PAM: Account Validation Failed - Rejecting User LPFRONTDESK-2$!
[2012/06/16 17:54:52.075880, 0] auth/pampass.c:797(smb_pam_accountcheck)
  smb_pam_accountcheck: PAM: Account Validation Failed - Rejecting User LPFRONTDESK-2$!
[2012/06/16 17:54:54.985320, 0] auth/pampass.c:797(smb_pam_accountcheck)
  smb_pam_accountcheck: PAM: Account Validation Failed - Rejecting User LPFRONTDESK-2$!

I've noticed that it does not record anything when I get the "There are currently no logon servers available to service the logon request." error (which makes sense, if the client computer cannot even find the PDC).