Yup, you really gotta make sure it's iPhone 3G compatible, not just iPhone (If you've got the 3G anyways.) as they changed the pins slightly I think. I think one of the reasons most don't work is because it uses the 5V Data cable on the USB as well, not just the 12V line to charge it.
I use an Alpine CDE-104BTi and it works fully with my iPhone 3G, both the bluetooth Parrot part and the playing.