To further add to the confusion, the basic snap-in adaptor and the media snap-in adaptor are both available now in the UK for the iPhone 3G and 3GS, but only the basic snap-in adaptor is available for iPhone 4, but you can't order it online. iPhone 4 media adaptor expected next month.
I have the basic snap-in adaptor for iPhone 4 installed and waiting. I've given up on music from the phone. I have a 16GB iPod nano, 12GB of HDD in the car and also a 16GB USB stick spare. More than enough music choice to be going on with