For that matter, you might be able to design a home-builldable PCB that uses the Wiznet 811MH MODULE (less than $20) and get something SW compatible for about half the price of the official shield.
Did you see the NKC electronics announcement recently? They've got exactly that.
I wonder why NKC limit the size of the PCB, I think it it were a longer then most shield could fit on top of it.
The idea was to align the Ethernet port with the USB port, on the outer side of the board. Originally I designed the PCB to be the same size as the Protoshield (still not as large as the full Arduino board), but it could have been a little bit better... but I saw a lot of empty PCB space and it looked much better a smaller board.Maybe for Version 2...