mellis:
BTW, there are generic Client, Server, IPAddress classes in the core, that various implementations can inherit from to provide a common interface. It probably doesn't share as much as it could but it's something.
I can see how the libraries could be designed to use the same function calls, but different low level libraries. It would be great if you could compile for an ENC28J60 just by changing the include file. Is there enough reason to add that? Does the ENC28J60 offer advantages the W5200 does not?
This is just a transport, not a protocol. Setting up the transport should be rather universal. I mean, how much is there to it? mac, ip, subnet, gateway, dns, dhcp.
Thanks for all your help, David. Your efforts do not go unnoticed by at least some here. 
Karma +1 for you.