Because GPS BT units communicate using the serial profile and that is not currently supported by iPhone. Hopefully this is just sw and will be in future releases. I bought a $50 holux M1200 and it works great, gets GPS signals clearly inside my house. Much more sensitive then the 4 year old Delorme EarthMate I had (the usb version). The Holux has a micro-usb port which it can power off of (and charge its batteries) from any usb source, but needs a custom cable to pass data over usb (but why bother). So stashing one of these in your car with a 12v to usb charger would let your iPhone run a GPS app anytime you were in the car. That would work fine for me.
Yep - I use one of these also with my Pocket PC running TomTom 6. I love my iPhone and would love to ditch the pocket PC.
I'm hoping that this scenario works out and that Google develop some iPhone software for speech output of directions. Since the map data Obviously this would require a constant data connection - so if TomTom are developing mapping software for the iPhone I would happily buy that!
Can't really see why we can't use a BT GPS receiver - makes logical sense really.