If you encountered the below error after you have joined your samba as a member to the Windows 2008R2 AD :
[2011/07/27 09:31:04, 0] rpc_client/cli_pipe.c:rpc_api_pipe(790) rpc_api_pipe: Remote machine your_WINDOWS_DC pipe \NETLOGON fnum 0x4returned critical error. Error was NT_STATUS_PIPE_DISCONNECTED
It is a samba bug, upgradeto samba 3.4.x or above