The DigiX design (schematic, layout, concept) is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License: http://creativecommons.org/licenses/by-sa/3.0/
For personal use - yes without a doubt! We'll even provide EagleCAD files to make it easy!
To sell or give away - yes you can, but we request that you make your offering unique in some way, and you must give it its own name (see the next section).
Possibly, Digistump LLC retains all rights to the DigiX name and logo as well as the Digistump name and logo. If you'd like to make an official DigiX product please contact us at support (at) digistump (dot) com. The DigiX name and logo (or derivatives of them) may not be used without express written permission from Digistump LLC.
Why? Direct clones are a challenge for any open source company, because not only do they introduce an inferior product that may damage the reputation of the original, but they also tax the resources (support requests, angry customers with clones, clones being returned) of both the community and company. We're a small company, there are just two of us, so we've decided (like Arduino) or protect the name of our products to ensure that DigiX and Digistump branded devices are authentic, well supported, and you can buy them knowing you are supporting future Digistump creations.
These restrictions apply to shields as well - however - in the case of both shields and development board derivatives, you are welcome to mention that your device is DigiX compatible.
You are also free to use the DigiX name in reference to our devices in tutorials, project write ups, etc. You may not imply that your product, project, or work is affiliated or endorsed by Digistump LLC or the DigiX team in any way.
Contact us: support (at) digistump (dot) com - we'd love to work with you!