Garmin finally gets it with StreetPilot, an iPhone app that does turn-by-turn navigation. From Engadget:
It undercuts TomTom’s $49.99 offering for iOS, while offering real-time traffic data, voice commands, lane assist, address book integration so you can navigate directly to your buddies’ addresses, and multitasking support.
Software GPS navigation. The only hardware is the iPhone. The only problem is that Garmin’s maps won’t display if your iPhone doesn’t have a signal.