I made a test board (single sided) using toner transfer and it came out well, except that I printed it back to front (doh!) so I had to solder most of the components bent leg style on top of the board. I have already noticed a few modifications that could be made.
1. I replaced the diode with a wire jumper (thanks Matseng).
2. I should plan on using stackable female headers meaning that I need an extra row of 4 male headers for the programming jumpers.
3. The Uno R3 has 2 extra pins on the bottom right (IOREF and one other, not connected) that probably need breaking out upstairs.
4. There is room on the right hand extreme of the board to break out the ISP6 header from the Arduino board below. (Although I can't say I have ever used it.)
5. Perhaps more useful is to break out the Arduino reset button on the shield.
6. It would be nice to break out GND and +5V in or near the prototype area on the double sided board.
7. For the speaker terminals, 5mm pitch terminal block is probably a more commonly used size than 3.5mm.
Programming seems to work with ArduinoISP sketch uploaded and avrdude -b 19200 -c avrisp -p m328p ... but I wonder if some people might prefer the usual ISP6 header. That way you can program the shield with an external programmer without having to change the firmware on the Arduino. Any thoughts on that? It would squeeze area available for prototyping so I don't want to do it unnecessarily.
Also I am going to make the default chip the atmega328p instead of the atmega168. The difference is only 20 cents and the extra flash memory might be useful to futureproof the firmware.
For the double sided board I am considering shrinking the board to 5x5cm for cheaper prototyping as Matseng demonstrated that this would work acceptably (http://seeedstudio.com/forum/viewtopic. ... b6aed44b75