I found this bug when I upgraded from 3.0.2 on Fedora1 to 3.0.6 from fedora
updates. It work on samba 3.0.2 and 3.0.4 but stop on 3.0.6

There is problem that user from windows system is connecting to samba with wrong
gid (99-nobody)

here are logs:

samba 3.0.4
[2004/09/08 09:55:49, 1] smbd/service.c:make_connection_snum(705)
master (192.168.32.245) connect to service tele2 initially as user GBS\test123
(uid=10060, gid=10015) (pid 29004)

samba 3.0.6
[2004/09/07 17:49:03, 1] smbd/service.c:make_connection_snum(648)
  master (192.168.32.245) connect to service tele2 initially as user test123
(uid=10000, gid=99) (pid 4735)
            ^^^^^^^

this is why user can't open folder. Permission to open directory is only for
gid=10015

samba 3.0.6 winbindd.log:

[2004/09/08 00:41:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist
[2004/09/08 00:42:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist
[2004/09/08 00:43:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist
[2004/09/08 00:44:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist
[2004/09/08 00:45:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist
[2004/09/08 00:46:00, 1] nsswitch/winbindd_group.c:winbindd_getgroups(1059)
  user 'root' does not exist