I would not classify the Arduino as a true SBC, at least by my definition and in the spirit of the original concept. Why? Because in the original examples starting in the 70s a SBC was a standalone device that was programmed via input switches or at best via a dumb terminal using a rom based monitor program.
The Arduino would not be possible without a PC to program it. So while the AVR micro-controllers are really great and all, they have to stand on the shoulders of the PC to actually utilize.
Lefty