The reference designs (Eagle files) for the Arduino Diecimila and Arduino BT are now available under a Creative Commons Attribution Share-Alike license. This is effectively the “source-code” for the boards: both the schematics and PCB layouts in fully editable format. Together with the Eagle Layout Editor (available in a freeware version), it’s all you need to get started making your own custom Arduino hardware.
We’ve also posted a document called So you want to make an Arduino? – a guide / policy for making your own hardware. While we hope to see many people creating their own versions of Arduino (and many already are), we’ve tried to set down a few guidelines to help ensure the success of these efforts and the continued growth of the project as whole. Please take a look. We don’t claim, however, to have figured this all out, so consider this policy as a first draft, and post your suggestions and criticisms in the comments.
We hope you have fun with the reference design and we’re looking forward seeing what you all come up with!