If you are forced to use services like open dns you have the risk that the router propagates the DNS servers to your machines. As the implementation of SAMBA in Leopard effectively blocks the MAC to be a master browser for performance reasons you end in a situation in which tiger machines connect and leopard ones don't see a single share. Let's fix it.
1. Go into System Preferences->Network then advanced DNS if you see here the IP addresses of the open dns servers this is the case. Unfortunately you will be forced to change the hosts file.