TechNerd, aka Glenn Fleishman, wrote a comprehensive  round-up of GPS navigational apps for the iPhone over at Macworld.

I hardly know what he's talking about (and have no idea what iPod's have to do with GPS apps, but here's Glenn totally nerding out):

Controlling music

iPod control is also an oddly important part of GPS navigation for anyone who routinely listens to music or podcasts while driving. You don’t want to switch out of a program to use the iPod features, and I have found the double-Home-button press to bring up floating iPod controls doesn’t work reliably, sometimes exiting the currently running app unless you press it just right. (All the programs I reviewed, when relaunched with a destination selected, either automatically reusme the route in progress, or prompt you to resume the route.)

GPS Apps: iPod control

Software	iPod control	Music selection	Podcast selection*	Voice over/ pause
AT&T Navigator	Weak	Playlist, artists, songs	No	Pauses for navigation
CoPilot Live	Good, but must navigate via Menu to start playing music	Full library selection	No	Voice talks over music
GoKivo GPS Navigator	Superb controls	Full library selection	Yes	Pauses for navigation
iGo My way 2009	Good	Full library selection	No	Voice talks over music
Magellan RoadMate 2010	Good	Full library selection	No	Voice talks over music
MobileNavigator	Good	Full library selection	No	Voice talks over music
MotionX GPS Drive	Excellent, although hidden in list view	Full library selection	Yes	Voice talks over music; optional paused playbck
Sygic Mobile Maps	None	-	-	Voice talks over music
TomTom	Poor: can't start or select music	No	No	Pauses for navigation; sometimes auto-resumes

*Most navigation software can't select a podcast for some reason, but all programs with iPod control can continue playing a podcast selected before the app was launched, including pausing and resuming it.

iPod control also varies whether you’re using an auxiliary input jack on a car stereo or “head-end” integration via a dock connector. Sygic is the only app to lack an integral iPod control. The rest vary from forward, back, and play/pause controls to full selection via an iPod sheet as if you were in the iPod app. All of the apps with iPod control will play podcasts that were selected and are already playing; only GoKivo and MotionX allow podcast selection in the app.

GoKivo offered the best iPod control.

Five of the programs speak right over iPod playback, and did so regardless of how the iPhone was connected to the car stereo. The other four programs, which pause iPod playback before speaking, automatically restarted the audio when connected via the headphone jack, but audio had to be restarted manually when I connected them via the dock connector to an integrated stereo system. TomTom paused the stereo, and in some driving sessions would auto-resume, but in others would not.

AT&T Navigator was particularly irritating, because the program is talky: it tells you quite a bit about what’s going on, with no controls to make it less prolix. I was forced to constantly press the resume button on the stereo to keep iPod playback going.