I've installed Samba3 from MacPorts. (following this guide) To run it I execute sudo /opt/local/sbin/smbd -D && sudo /opt/local/sbin/nmbd -D in a Terminal window and everything works fine.
I've attempted to create a LaunchAgent to run samba with the system, rather than having to invoke it manually. My plist file is called com.samba3.plist and consists of the following:
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.samba3</string>
<key>ProgramArguments</key>
<array>
<string>/opt/local/sbin/smbd</string>
<string>-D</string>
<string>&&</string>
<string>/opt/local/sbin/nmbd</string>
<string>-D</string>
</array>
<key>RunAtLoad</key>
<true/>
</dict>
</plist>
However, on system startup samba is not being run.