There are only two subscription GPS apps on the iPhone right now:
AT&T Navigator and Gokivo
They sure as heck won't change from that model anytime soon, and TomTom won't convince them to change from that model either. Sygic, Navigon, and XROAD all are one-time fees, download maps to the device, and work okay for navigation.
The reason why I say they won't change is because the
why they charge a subscription is unlikely to be threatened in the near term unless TomTom does something very out of character.
See, AT&T Navigator includes local search over the cellular network, traffic, gas prices, ratings of restaurants and so on. It even ensures you always have the latest maps because they are streamed over the cell network.
Historically, that sort of live feed requires a subscription of some kind in the stand alone units (MSN Direct for Garmin, and the new Live services, or some of the Home services for TomTom). Don't forget you have to pay for map upgrades on the stand alones.
If TomTom's model is anything like the model for the standalone (maybe some discounted pricing on the live services or included in the price if they just really want your business)... you will still need to pay for the map upgrades, whereas the AT&T Navigator will always use the latest map data they have paid for. The flip-side is that TomTom's model is also the one that will work out of range of a cell tower, whereas AT&T Navigator won't.
As I will be travelling outside of cell range on national park grounds, the TomTom app, or a stand alone unit is what I have to use. Because of the timing of everything, I wound up getting one of the nice 4.3" TomTom units. Now if I could write up a script to scrape AT&T WiFi hotspots off their website and create a TomTom POI DB, I'd be set.