There's a reason why more people haven't seen the next iPhones before Steve Jobs makes an announcement: they're in disguise.
This iPhone, which looks drastically different from the 3GS, was enclosed in a custom-moulded plastic case so it could be used in public without attracting attention. In regular use, you would think that this was just a standard iPhone with the writing scratched off the back. Very clever.
The plastic case, which comes apart easily, looks just like a 3GS. When you pry the case apart, three bits - the power button, the mute switch and the volume rocker - quickly shed off. It's weird that these bits are made of plastic, when the corresponding parts on the 3GS are made of aluminium/metal.
To reassemble the case, all you have to do is make sure the little plastic bits are in the right place before popping the front back on. A very ingenious solution to protect future designs from lookeyloos. One of the best bits is that the case looks like a case FOR an older iPhone. iPhone cases are seen so often, that even if this one looks weird and doesn't match up to the 3GS body, it can be easily dismissed as just being a lousy case.
The Complete Lost iPhone Saga