I have a project that is currently in pre-production testing that has a few custom boards (which are being turned in a single I/O PCB) and a WIZnet WIZ811MJ plugged into a MEGA using the shield kit from adafruit.
To enable the box to be as small as possible (and because the piggy-back approach isn't really suitable for production purposes) I'm considering taking the MEGA reference design & adding a W5100 chip directly onto the board.
Although I don't need to keep the dimensions even remotely similar to the Arduino ones for my own purposes, I thought that other people might be interested in a board with this facility.
1] Remove the USB port & controller chip as a means to claim a fair amount of the space needed. 2] Change the voltage regulation circuit so that the whole thing can run properly off a 9V power supply (7, 7.5 and 8V power supplies are difficult to get hold of.