Well the Moto 360 received a 3/10, citing the replaceable bands as a positive, but apparently the battery was quite a bit less manageable to remove. I'm betting this will receive a 3 or a 4.
I was wondering the same thing! I assume that's exactly what it is, since the watch will put the screen to sleep if you cover it with your hand, but I would also assume that the AMOLED display would interfere with the sensor.
Definitely looks like one. I'd venture a guess that it's not much different than barcodes/data matrices found on other processors, likely acting as an identifier of some sort (plate no., serial no., lot no., etc.).