I tried to upgrade my 1000+ Users/Machines installation from 2.2.7a to 3.0.13.

- after upgrading *ALL* Windows NT4 Clients (SP4 to SP6a) failed to logon to the
domain. All WindowsXP-clients worked well.

- Removing the NT4 clients from the domain and readding them only partially
solved the problem, since the User where no longer able to read their former
registry settings from ntuser.dat.

Step by Step:

Upgrade to samba-3.0.13 with the follwong changes to the smb.conf:
first tried with: 
    passdb backend = smbpasswd:/var/samba/logon/private/smbpasswd
  then moved to: (used pdbedit to convert the smbpasswd)
    passdb backend = tdbsam:/var/samba/logon/private/smbpasswd.tdb 
  without noticeable changes.

  used:    
    dos charset = CP850
    unix charset = CP850
  to retain character-set compatiblity !

Here is my smb.conf file of the logon-server:
[global]
   workgroup               = testdo
   netbios name            = logon
   domain logons           = yes

   bind interfaces only    = true
   interfaces              = 127.0.0.1/8 \
                             192.168.18.29/32

   log file                = /var/samba/logon/log/%M.%m
   lock directory          = /var/samba/logon/locks
   pid directory           = /var/samba/logon/locks
   max log size            = 2500
   log level               = 1
   syslog                  = 1
   utmp                    = false
   utmp directory          = /var/adm

   time server             = yes

   load printers           = false
   encrypt passwords       = Yes
   passdb backend = smbpasswd:/var/samba/logon/private/smbpasswd
#   passdb backend = tdbsam:/var/samba/logon/private/smbpasswd.tdb

   security                = user

   os level                = 64
   local master            = yes
   domain master           = yes
   domain admin group      = +winadmin
   preferred master        = yes

;   wins server             = samba1
   wins support            = yes
   wins proxy              = no

   default case            = lower
   preserve case           = false
   short preserve case     = false
   #mangle case             = no
   case sensitive          = no
   dos charset = CP850
   unix charset = CP850

   oplocks                 = false

   nis homedir             = true
   homedir map             = auto.pchome

   logon script            = %M.bat

   logon drive             = Z:
   logon home              = \\%N\%U\profiles\%a
#  logon path is used by NT
   logon path              = \\%N\%U\profiles\%a

#============================ Share Definitions ==============================

[netlogon]
   comment                 = Network Logon Scripts
   path                    = /var/samba/netlogon
   guest ok                = Yes
   writable                = no
   browseable              = yes
   root preexec            = /var/samba/etc/mklogon.sh %U %M %a %N %I
   root preexec close      = true
   root postexec           = /var/samba/etc/rmlogon.sh %U %M %a %N
   write list              = @winadmin