It would not be unethical for Apple to do that. People are doing their best to get their hands on confidential prototypes and early info, it is fair for them to do whatever it takes...including producing a fake iPhone, in order to keep people off the trail. Give them slow but consistent leaks through an unknown source and everyone will be looking for that.
Is it possible to keep under wraps? I think so...it just depends on how far they want to go to make it happen. Regarding leaks during the manufacturing process, the workers are on total lockdown, working in a highly monitored setting and living in dorms. Not only that, they know they face prosecution if they make off with a part. It would be very hard and very risky to try to get something out.
I guess will know in about a month if Apple was successful in keeping things under wrap...