I have a problem joining samba 3.0.26a to ADS on Windows 2003 R2 SP2.
Issuing the same procedure with a clean Windows 2003 Server Service Pack 1 install I can join my linux box without any problem. The problem appears only on Win2003 server with all modern updates installed. Kerberos authentication seems to work correctly and linux box receives a ticket:
# klist
Ticket cache: FILE:/tmp/krb5cc_0
Default principal: administrator@TERMINAL.B2MOTOR.RU

Valid starting     Expires            Service principal
10/08/07 12:28:59  10/08/07 22:30:09  krbtgt/TERMINAL.B2MOTOR.RU@TERMINAL.B2MOTOR.RU
        renew until 10/09/07 12:28:59


Kerberos 4 ticket cache: /tmp/tkt0
klist: You have no tickets cached

Though when I try to join samba in AD domain I got the following message on stdout:
[2007/10/08 12:31:58, 0] libsmb/smb_signing.c:signing_good(253)
  signing_good: BAD SIG: seq 1
[2007/10/08 12:31:58, 0] libsmb/clientgen.c:cli_receive_smb(140)
  SMB Signature verification failed on incoming packet!
[2007/10/08 12:31:58, 0] libsmb/cliconnect.c:cli_session_setup_blob(586)
  cli_session_setup_blob: recieve failed (NT_STATUS_INVALID_PARAMETER)
Failed to join domain: Invalid parameter

Here is my smb.conf:
[global]
	dos charset = cp866
	unix charset = koi8-r
	display charset = koi8-r
	workgroup = TERMSRV
	realm = TERMINAL.B2MOTOR.RU
	server string = Proxy Linux
	security = ADS
	auth methods = winbind
	password server = terminal.terminal.b2motor.ru
	passdb backend = tdbsam
	log file = /var/log/samba/log.%m
	max log size = 50
	socket options = TCP_NODELAY
	preferred master = No
	local master = No
	domain master = No
	dns proxy = No
	idmap uid = 10000-20000
	idmap gid = 10000-20000
	winbind separator = +
	winbind enum users = Yes
	winbind enum groups = Yes
	winbind use default domain = Yes
	cups options = raw
	case sensitive = No