I've completed and soldered my heater board.
There are 8 PWM outputs from my reprap shield which are boosted using 54A MOSFETS. These should happily drive my extruder heater, heated bed, fans, extra motors, whatever.
I've posted the files to thingiverse. I've also added my single-sided stepper driver files to a thing as well.
I now have four working stepper drivers, one mega shield, and a completed heater PWM board. Only a couple of thermistor sensors, and I'll have a complete set of electronics for a mendel :-)