The samba service is running on MacOSX 10.4.6
I'm adding the log, maybe this will help you [to me it looks like nautilus closes the connection for some reason]. Maybe somebody with a mac can also reproduce this problem?

[2008/03/25 15:41:09, 2] /SourceCache/samba/samba-100.4/samba/source/passdb/pdb_interface.c:make_pdb_methods_name(654)
  No builtin backend found, trying to load plugin
[2008/03/25 15:41:09, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/pdb/opendirectorysam.so' loaded
[2008/03/25 15:41:09, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/auth/opendirectory.so' loaded
[2008/03/25 15:41:09, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [99]
[2008/03/25 15:41:09, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/vfs/darwin_acls.so' loaded
[2008/03/25 15:41:09, 1] /SourceCache/samba/samba-100.4/samba/source/smbd/service.c:make_connection_snum(648)
  192.168.101.55 (192.168.101.55) connect to service imjv initially as user unknown (uid=99, gid=99) (pid 24662)
[2008/03/25 15:41:09, 0] /SourceCache/samba/samba-100.4/samba/source/smbd/service.c:set_current_service(51)
  chdir (/Volumes/data/imjv) failed
[2008/03/25 15:41:13, 2] /SourceCache/samba/samba-100.4/samba/source/passdb/pdb_interface.c:make_pdb_methods_name(654)
  No builtin backend found, trying to load plugin
[2008/03/25 15:41:13, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/pdb/opendirectorysam.so' loaded
[2008/03/25 15:41:13, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/auth/opendirectory.so' loaded
[2008/03/25 15:41:13, 1]
auth_ods.c:opendirectory_smb_pwd_check_ntlmv1(387)
  opendirectory_smb_pwd_check_ntlmv1:
[0]opendirectory_user_auth_and_session_key key_length(0)
[2008/03/25 15:41:13, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [20]
[2008/03/25 15:41:13, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [1027]
[2008/03/25 15:41:13, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [1029]
[2008/03/25 15:41:13, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [1025]
[2008/03/25 15:41:13, 1] pdb_ods.c:odssam_getgrgid(2870)
  odssam_getgrgid: gid [1026]
[2008/03/25 15:41:13, 2] /SourceCache/samba/samba-100.4/samba/source/auth/auth.c:check_ntlm_password(360)
  check_ntlm_password:  authentication for user [rvandyck] -> [rvandyck] -> [rvandyck] succeeded
[2008/03/25 15:41:13, 2] /SourceCache/samba/samba-100.4/samba/source/lib/module.c:do_smb_load_module(63)
  Module '/usr/lib/samba/vfs/darwin_acls.so' loaded
[2008/03/25 15:41:14, 1] /SourceCache/samba/samba-100.4/samba/source/smbd/service.c:make_connection_snum(648)
  w2jzvbhb01 (192.168.101.55) connect to service imjv initially as user rvandyck (uid=2596, gid=20) (pid 24663)
[2008/03/25 15:41:14, 1] /SourceCache/samba/samba-100.4/samba/source/smbd/service.c:close_cnum(836)
  w2jzvbhb01 (192.168.101.55) closed connection to service imjv
[2008/03/25 15:41:14, 2] /SourceCache/samba/samba-100.4/samba/source/smbd/server.c:exit_server(595)
  Closing connections
[2008/03/25 15:41:15, 2] /SourceCache/samba/samba-100.4/samba/source/smbd/server.c:exit_server(595)
  Closing connections
[2008/03/25 15:41:15, 1] /SourceCache/samba/samba-100.4/samba/source/smbd/service.c:close_cnum(836)
  192.168.101.55 (192.168.101.55) closed connection to service imjv