The EtherSex firmware supports packet forwarding quite well in the meantime. This is you can use an EtherSex as a router to attach a RFM12 or ZBus network to the normal ethernet network. With ARP-proxying and IP forwarding enabled all the clients on the ethernet wire are capable of accessing any RFM12 device transparently.
With RFM12 or ZBus you are able to indirectly attach (many) smaller controllers to the network (at zerties.org we use the ATmega8 regularly). If you do this with IPv6 you can give your ATmega8 a worldwide unique IPv6 address, reachable from everywhere :-)
To configure a bridge and its slave network you need to do the following: The whole subnet will have the same protocol (IPv4 or IPv6). So when your bridge is IPv6 all slaves must also be configured for IPv6.
On the "router" you first have to choose the normal setup and configure it as usual at first. Then enable RFM12, USBnet or ZBus (or any combination) and provide non-overlapping address ranges to each enabled interface. Last but not least, enable IP forwarding in the network configuration tab.
For the "slaves" you have to pick a target profile like RFM12-Teensy and then configure like it would be a normal EtherSex attached to ethernet.
After you have compiled and flashed all your controllers, you have to add network routes directing the traffic, thought for the slaves, to the EtherSex acting as the router. On Linux you do it like this:
ip route add 192.168.1.0/24 via 192.168.0.10
192.168.1.0/24 is the subnet and 192.168.0.10 is the ethersex's IP address.
Besides that, if you have a router with configured RFM12 and ZBus interfaces, any device residing on the RFM12 side will be able to send data to any ZBus device (and the other way round of course).