For Mac OS X 10.4.10+ Tiger (Samba 3.0.10) trick was to specify guest account and security. For sharing read/write, uncomment last line.
[global]
guest account = unknown
security = share
server string = skynet
netbios name = skynet
guest ok = yes
oplocks = yes
[data]
path = /data
# read only = no
For Mac OS X 10.5 Leopard (Samba 3.0.25b-apple) remove security line:
[global]
guest account = unknown
server string = skynet
netbios name = skynet
guest ok = yes
oplocks = yes
[data]
path = /data
# read only = no