I started out with FEZ devices like the Domino and Panda.
The Arduino form factor is a great starting point for prototyping and testing. One could wire stuff up to a proto bard or a bead board and get things working before one started schematics and PCB layout of the actual device.
With the USBizi chipset you could make a high quality device based on the GHI electronics NETMF port and maybe sell it in small quantities at sites like https://www.tindie.com/
This way of working seems to be history now and Im wondering what is the path from a .NET Gadgeteer set up to a complete standalone device?