I've spent a little time putting together a tutorial for producing an Arduino Shield PCB from start to finish. I'm hoping it'll be a useful resource for people wanting to extend their Arduino's
Thanks, that's a very detailed tutorial and therefore really helpful.
I'd just like to point beginners to the alternative of using Fritzing. It's really made for quickly creating Arduino shields, if your demands are not too advanced. There's a tutorial at PCB design with Fritzing.
I've done something similar for the Pulsonix software I use. I imported the Arduino Eagle Duemilanove files into Pulsonix, extracted the board outline, connector positions and mounting hole positions, and saved them as a PCB profile file. It only took a couple of minutes: