or... Tim Cook in a photo, with one hand over his other hand/wrist, for no special reason...
Seriously: do you think that, at a company that takes future product secrecy too seriously - and of all the employees at that company, the one that specially enforces that secrecy - would go around his day with a prototype of a new product, and/or forget to take it off before a staged picture for the press?
I don't think so...
But if you like conspiracy theories, I'd be drawn to go the other direction... he is purposely covering his wrist to stir up speculations about an iWatch, and boost the APPL stock price.