I've just started a video series on electronics product design! This might be useful to anybody who has that special idea, but needs help acting on it. I use an AVR robotics board as an example (so it's quite relevant to Arduino), and I plan to document it right through from start to finish!
Part 1 is here: http://www.youtube.com/watch?v=qfQuPIJPGaA
Part 2 is coming soon, just waiting on some MCUs from Atmel :) Subscribe to me on YouTube to follow it! I also have quite a few Arduino tutorials, as well as just regular electronics tutorials (soldering, PCBs, etc.) so check out my channel!