The new Tom-Tom app has been released for iPhone. Read about it here.
I've been using the AT&T nav app and it is not particularly good. I'll post a review of it soon. $100 is a bit steep for the Tomtom app, but it is a lot less than buying a GPS device and you only pay it once, so that's not too bad. Plus, I've used both the Garmin Nuvi and own a standalone TomTom and they are both quite good from an interface standpoint.
Downside is the screen size, but if you can put it on your car screen, that would be great. Runs in both portrait and landscape mode, which is nice.
I've bee able to determine that the new 3.0 software *does* work with the RCA input of my car screen, but ONLY for video playback. The ScreenSplitr and TVOut apps are broken for it.