samba - Apr 2004 - SMB signing is mandatory: where to enable it?

While attempting to use cifs to mount a win2k3 share, I get this error:

[root@dhcp190 root]# mount //win2k3/data -t cifs -o username=administrator
/mnt/smbfs
cli_negprot: SMB signing is mandatory and we have disabled it.
2687: protocol negotiation failed
SMB connection failed

Where exactly do I enable smb signing?

kernel 2.6.5 + samba 3.0.3