The new iPhone might make sense if one does not own an iPod and GPS system and is about to upgrade one’s phone.
The iPhone is going to “know where you are” according to Apple, probably not through GPS but through data it receives from the phone network. Though Apple didn’t show off navigation software, it did show off integration with Google Maps, and so the iPhone represents a reasonable alternative to a GPS system, unless you really need turn-by-turn navigation.
Putting together the cost of an iPod and a GPS unit might be enough to persuade you that the iPhone represented value for money, though one should also note that a GPS service will not incur additional cost for each use, whereas Google Maps is going to eat up your data plan.