It has come to our attention that there is another issue with Windows 2K8 R2.
In certain scenarios Kerberos authentication fails in addition to NTLM based authentication.
You will see the following error in /var/log/samba/log.winbindd
cli_pipe_verify_schannel: auth_len 56
There is currently no workaround for this issue. Please escalate the case so that we can update the internal bug count and let the customer know that we are currently investigating the issue.