I believe the OP is discussing the bluetooth enabled pucks that are typically under $100. On most other smart phones you can use the serial port support in the bluetooth stack to get GPS data onto a phone that does not support it internally. - Most Treos are like this. The mappign utility then uses the GPS data via bluetooth to plot your position and the software can then do the turn by turn directions most people keep asking for. I personally am hoping that Apple will let us do this in a future software update.
If they enabled BT GPS' to work with Google Maps, I'd be first in line to buy a GPS "puck" for my car. That would be a very sweet addition and very simple addition, not to mention one more way Apple and Google can work together.
I enjoy Google Maps as it is right now, being able to go further w/ them would be the ultimate.