In case you were not aware, TomTom and Magellan have previously released versions of their GPS car kit that turns your iPod Touch into a navigation device, as well as making their GPS software compatible with it. One of the downsides is the cost, as it can be argued that you can buy a standalone GPS unit for much less. Cost may not be an issue for those wanting to add more functionality to their device but one of the drawbacks I see is that their car kits are for in car use only via the car power adaptor. In my opinion, this limits some of the payback use / value, especially if you only need GPS navigation occasionally.
