I've been working for a couple of weeks on a RepRap Arduino Mega shield. I've got a mix of early generation 2 electronics, a few home-made single sided generation 3 stepper boards, and old arduino with a screw-terminal shield and a brand-new arduino mega.

I designed myself a version of an arduino mega shield to connect the gen3 stepper boards using the 10-pin IDC cables, and it might be useful for anyone transitioning between generations of electronics. Features:

• Single-sided for PCB for easier home-made boards
• 5 stepper driver headers - x,y,z,a,b - should match existing gen3 electronics.
• (future-proof?) UI interface header that *should* match gen4 electronics.
• Heater 10-pin IDC (to plug in MOSFET driver board later) - should be able to drive a reprap setup with 1 or 2 extruders/ heated bed without an additional extruder board.
• I2c header as per standard boards
• Optional extra 3-pin headers for digital in\out and analog in : +5v, GND, and data pin. This makes it easy to add temperature sensors, kill switches, extra limit switches, etc (I'm only using the min endstops).

You can treat this as more of a prototyping board, and solder as many headers/pins as you need: I bought a dozen 50-pin headers, so I filled mine up. I'm only expecting to use one or two of the analog inputs as temperature sensors for now.

NOTE that the +5v/Gnd/Signal headers for the digital and analog pins are directly connected to the internal arduino 5V supply. DO NOT run anything major off these headers. I was only planning to add simple data switches (digital input from high/low), simple thermistor circuits (Analog input) and potentially a small servo (9g micro servo) off these I/O pins - so I didn't pay much attention to them. If you are running any further circuits, use a separate power supply! (all RepRap boards have separate 12v PSU headers anyway).

My build went well. I 'tinned' the copper pads (using some plumbing flux and a tiny bit of solder), which made the soldering a bit easier. Using a tip (from ladyada?) I converted some standard header pins to some through-pins by pushing them through using the flat on a pair of pliers, three or 4 at a time.

Once I'd lengthened the pins, I could put them through from the top of the board into the arduino and check the alignment. I left the pins partially pushed in to maintain the position, and soldered a single pin from each row.
This held them in place when I removed the mega and soldered the rest.

Top Tip : apply the silkscreen layer before you solder the board up. I didn't and had to write all my pin numbers on with a sharpie.

Some pictures of my build in progress:
http://renoirsrants.blogspot.com/2010/06/renoirs-mega-shield-built.html

If you're looking for the arduino mega shield, pins only, I've got some eagle files here:
http://sites.google.com/site/renoirsrants/reprap/arduinomegashield?pli=1

EDIT:
Updated with thicker traces and ground plane, and 0.3mm centre drills.
Should be easier to home-etch.