I may be preaching to the converted here and you may all know about this already, if so forgive my enthusiasm. But for those who don't know, if you are wanting to build your own PCB to connect with the Pi, please take a look at Seeedstudios' "Fusion PCB" service. It is AMAZING.
Story - I got fed up hand-wiring and messing up Humble Pi boards, and to build my Chamber Orchestra I need lots of boards - 6 copies of one board (Player) and one of another (Conductor). The hand-wiring was driving me bonkers, so I decided dammit, I was going to make a PCB. This was exactly 2 weeks ago. So I downloaded EAGLE, spent a Saturday getting to grips with it, spent a Sunday tuning the design I had, and shipped off Sunday evening the finished Gerbers after checking them online at http://www.gerber-viewer.com (I'm Mac-based so can't use any of the Win Gerber apps) . This is an honest timescale, it was literally one weekend, having never used EAGLE before and not having done a PCB design for over 25 years.
And exactly 2 weeks later, here is a manufactured PCB.
I know there are dozens of companies offering the same service - ship us Gerbers, we will ship you hardware - but the price is the killer here - I got 10 PCBs for $13. Not each, total. $1.30 EACH for 10 PCBs.
It is *REMARKABLE* and quite frankly, it's insane, but it's real. Now I'm no electronics engineer, so there will I'm sure be issues, but at this schedule and at these prices, anyone can free their inner sparky and just give it a shot, and iterate 4 or 5 times.
So, Seeedstudios Fusion PCB - http://www.seeedstudio.com/depot/?products_id=835 - I can't quite believe it's for real, but it is.