I got a 3G iPhone as an impulse buy the other day (they're plentiful in Oregon it seems, no lines, got one in ten minutes at 2pm on a Friday) but the big downside aside from the higher monthly cost was that my trusty old Belkin car adapter with pre-amp line out sound no longer charged the iPhone. The newest iPhone accepts power on the USB architecture instead of the old firewire one, so most every old car charger no longer charges your iPhone (and being the battery killer the 3G is, this is important to charge it as often as possible).
After getting the package and setting it up, I have to say it does exactly what it should, charging the 3G iPhone and transmitting sound via the line out cord into my car's aux-in jack. The extra cool bonus feature is that the wireless remote actually works on my iPhone, allowing me to now skip songs, pause playback, and toggle shuffle directly from my steering wheel without taking my eyes off the road. I really didn't expect the wireless remote (with included steering mount) to function on an iPhone, but it does and it's great.
So high marks all around for this. If you have a new iPhone, and you want to charge/listen to your iPhone in the car, this Kensington option is a good one (there's a $5 more expensive version with a iPhone holder on a stalk that I didn't need, but you might).