This incorrect. The hardware is capable of recognizing upside down, as in the end with the sleep wake button pointing down, but some software ignore this, such as safari. If you mean face down, then I think the iPhone is technically not able to tell this from face up, but well programmed software could make up for it. For example by what orientation the iPhone was in prior and the direction it was moving.
This incorrect. The hardware is capable of recognizing upside down, as in the end with the sleep wake button pointing down, but some software ignore this, such as safari.