Maybe It could interest you.
I'm working in a Slip to Ethernet bridge. I have the prototype almost finished.
The idea behind It is to connect my 'shortly' expanded A500+ (no accelerator card, and only 1,5 Mb of chip ram... by now) to my network, and to the Internet by extension...
Basically, It takes the incoming IP datagrams from the ethernet, makes some simple processing (discard bad packets, check the target IP, etc...), and resends them to the serial port in Slip format.
In the other direction (Serial to Ethernet), It takes the datagrams from the serial port, and send them to the net, taking care of things such the ARP and all those other 'nasty' things

At first, It will only support Slip protocol. I guess PPP protocol will be added in the future.
I'll post some photos as soon as I can.