I want one as well. When the iPhone supports A2DP that would be perfect.
I did find a couple products, the problem is it also uses it for the phone. We need a product that only does audio, and lets us still use our standard bluetooth phone connection. However, that means two separate bluetooth connections, which might eat the battery a lot (but hey, I've got an iPhone charger in my car...)
Saving the hassle of plugging in the audio port though would be great.
Motorola has a T505 (and a T605) for only like $70-80 at most places online.
http://direct.motorola.com/hellomoto/t505/
There is a Motorola D650 iPod adapter. I did not look to see if all this does is rebroadcast A2DP like the Sony below. (Which of course doesn't help as we need a receiver for A2DP too, duh!)
There is a Sony BT transmitter to transmit A2DP since iPhones don't do it yet.
Ideally we could find a Bluetooth A2DP with audio out and just plug it in to the aux port (and can charge via cig lighter) all inside of the armrest and never even see it.
Bonus: Something to actually control the iPhone/iPod. I'm going to ask tomorrow when I go in for service if they have any ideas on this. I've seen an aftermarket hack, but it seemed a little too crazy for me.