Thanks for your post. I wonder what it could be. Must be something to do with the wireless signals constantly being so close to the hand / wrist although it seems as if its only effecting a minority like ourselves. I was hoping a different band might help tackle the problem but haven't got one to test at the moment.
For info I have now tried on both arms and start getting a dull pain / discomfort from the wrist all the way to my elbow within about 30 mins of wearing it!
It's definitely not the WiFi/Bluetooth. Otherwise you'd notice pain in your hip too, or where ever you keep your iPhone.