The TomTom Car Kit is on the US Apple Store right now for $199.95 with free shipping. The catch is that the kit is listed as not shipping for 2 to 3 weeks. If you use your iPhone as a navigation device a lot, this may be a good item to get.
The TomTom kit is expensive, but it offers a built-in speaker, TomTom maps, a docking station, and more for your money. Nav apps suck some battery life if you don’t have the iPhone plugged into the power outlet while in use.