I looked for some eagle files for an arduino mega shield, but couldn't find any. Here's some I ripped from the Arduino reference design. It might give you a head start.

Renoir's MegaRap Shield

I built a shield for my own reprap electronics. It's loosely based on the generation3 electronics, and should allow a reprap to run off a single arduino Mega. It should be compatible with the standard makerbot stepper driver boards, and should plug in using the standard ribbon cables.
There is a (hopefully future-compatible) interface socket, which should match the new generation 4 boards. There are a couple of extra stepper sockets and a heater socket for output to a separate mosfet driver board.