I'm not sure Atmel AVR microcontrollers (used in the rest of the Arduino line) are open source hardware either. Sure, they are fully documented from the user's

I'm not sure Atmel AVR microcontrollers (used in the rest of the Arduino line) are open source hardware either. Sure, they are fully documented from the user's perspective, but Atmel doesn't give away source (Verilog/VHDL/etc.) for the core. (I'll give you that ARM is more patent-encumbered.)