I have been investigating the options on this too. Price vary wildly with features and functions, from $100+ to $500+. I was looking for something that would support an Iphone 3G myself. The rumor mill is that one of the future generations of Iphone will support music thru BlueTooth, so I am keeping that in mind. Supposedly the 2010 Toyota Crossover will support this feature.
One thing that intrigued me was being able to watch video off the Iphone. I did find a unit that would display the Iphone video on the front Nav but it was the high end solution. I would be interested in someone that has the NavTool installed.
NavTool
It did not seem to control the Iphone/Ipod must lists like the following solution.
I have also been look at the vaistech SL2Vi. This solution does not seem to support the video, but will enable the car to control the Iphone/ Ipod like it does the radio or CD, with song lists appearing and the steering wheel controls. But from what I can determine this is for only music stored on the Iphone/Ipod. Since I primarily use Pandora, I am not sure if it will be of ANY value.
SL2Vi
Other than that, I don't see much of a benefit from any of these solutions. I can plug the Iphone/Ipod into a $5 charger or a $3 audio cable and hear the music on the cheap.