Here's the first iPhone subscription app (enabled with the OS 3.0 update) I've seen so far...

AT&T Navigator 1.0i (iTunes App Store)

The app itself is free. However, using the app on the AT&T Wireless network costs $9.99 per month which appears on your AT&T bill. You need to go to att.com/directbill or call 800.331.0500 to cancel the subscription.

The app provides turn-by-turn navigation with a three-quarter view 3D-ish view and voice guidance. It also provides real-time traffic updates but does not say where those updates are available. In my experience, real-time traffic updates are limited to specific cities (not mine for example).

$9.99 per months seems a bit high ($120 per year + tax) when you can buy a nice dedicated GPS for between $150 and $200 these days (probably less if you look around a big). On the other hand, you don't need to carry another device just for directions? It only has 6 user ratings right now. But, those few reviewers seem very happy with the app giving it 4.5 (out of 5) stars.