I'm running with 'security = server' and everything was working fine with version 3.0.24 of samba.  I upgraded to version 3.0.25 and I could no longer access my shares.

My XP client now reports, "The system cannot find the device specified." and a client->server network capture shows a logon failure:

10.1.2.4  10.1.1.83 SMB Negotiate Protocol Request
10.1.1.83 10.1.2.4  SMB Negotiate Protocol Response
10.1.2.4  10.1.1.83 SMB Session Setup Andx Request, NTLMSSP_NEGOTIATE
10.1.1.83 10.1.2.4  SMB Session Setup Andx Response, NTLMSSP_CHALLENGE, Error: STATUS_MORE_PROCESSING_REQUIRED
10.1.2.4  10.1.1.83 SMB Session Setup Andx Request, NTLMSSP_AUTH, User: DOMAIN\user
10.1.1.83 10.1.2.4  SMB Session Setup Andx Response, Error: STATUS_LOGON_FAILURE

A server->domain controller network capture shows the following repeating sequence (note that the anonymous Session Setup is successful):

10.1.1.83 10.1.1.1  TCP 45010 > microsoft-ds [SYN]
10.1.1.1  10.1.1.83 TCP microsoft-ds > 45010 [SYN, ACK]
10.1.1.83 10.1.1.1  TCP 45010 > microsoft-ds [ACK]
10.1.1.83 10.1.1.1  SMB Negotiate Protocol Request
10.1.1.1  10.1.1.83 SMB Negotiate Protocol Response
10.1.1.83 10.1.1.1  SMB Session Setup Andx Request, User: anonymous
10.1.1.1  10.1.1.83 SMB Session Setup Andx Response
10.1.1.83 10.1.1.1  TCP 45010 > microsoft-ds [FIN, ACK]
10.1.1.1  10.1.1.83 TCP microsoft-ds > 45010 [FIN, ACK]
10.1.1.83 10.1.1.1  TCP 45010 > microsoft-ds [ACK]

Downgrade back to version 3.0.24 and the problem goes away.