Will the iPhone GPS Put A Dent In GPS Sales?

Daniel Lyons, aka Fake Steve Jobs, poked a little fun at a possible drop in Garmin sales. That got me to wondering if the iPhone with an integrated GPS really would have an affect on sales of other units. My wife bought a Magellan Maestro 3100 last year. I call it "Nagging GPS Lady", for the female voice that keeps telling to to turn when I either miss a turn or think I know a better route than the GPS recommends. I still enjoy that GPS. I've had GPS capability on my Pocket PCs for several years, and my current phone has a built-in GPS. Still, I keep my wife's unit locked in my car "in case I need it."
I can't predict the future, and I don't think I'd try, but I still think a regular GPS would be a great asset next to an iPhone. I'm sure somebody will make a car mount for the iPhone if one doesn't exist already, but I'm not sure how well it would work as a stand alone unit to drive with. I'm sure it's fine if you're walking around and trying to find a Starbucks, but how well would it work to find an address while you're driving?
I'm sure we'll find out by the end of next month.