I've built this now, and I'm feeling very pleased with myself, even though I screwed up one thing. I've gon into some tedious detail on my blog because this is the first SMD thing I've ever done: http://mrkimi01.blogspot.co.nz/2013/...ack-board.html
I've also put the final Eagle files and a test program at https://code.google.com/p/teensy3-piggyback-test/
Paul's suggestion of adding the capacitor didn't make it in yet and it seems to work okay. But if I plan to add it to the next version.

The one thing I screwed up was the voltage measurer, which is just a voltage divider feeding into A0 on the Teensy3. I got the resistors the wrong way around so it doesn't give me a useful reading. That was an error during the build rather than the design. So quite a dumb mistake.