However just how robust is this this little wonder in real life if it were to be put into a commercial product?
It's just as robust as any other board... It depends on what you mean, and what kind of electrical & physical environment you're talking about. I could kill an arduino in an instant by applying excess voltage, and I wouldn't expect it to last too long if exposed to sea water.
About 20 years ago I built a car alarm with a different microcontroller. It's still running, and the program "runs" 24/7, armed or not. It never "crashes" and the only time it gets rebooted is when the car battery dies. I'd expect the same reliability from the Arduino. Or even better reliability, since the Arduino is built on a PC board and my old alarm is built on a "breadboard".
I wouldn't send the Arduino into space or use it in a life-support application, but I'm not an expert in those high-reliability fields and I wouldn't use anything I built in an application like that.
If inputs/outputs are exposed to the "outside world", you can add [u]protection diodes[/u] to the inputs, and series resistors to the outputs, etc. There is the [u]Ruggeduino[/u] which has some electrical protection, and there is something called the [u]Industruino[/u] (which I don't know anything about).