Hi -
I have the 'official' lead in the GT and it works fine just doesn't display any info on the radio display. Haven't spent any time working out if it should though.
In our Volvo I use the Parrot MK9000i
Parrot - Hands free systems which does bluetooth handsfree, streams audio from the phone and you can also plug in an ipod etc.
It works and sounds fine but actually uses it's own little amp to supply the sound direct to the speakers so if you are a real audio nut it may not be good enough for you.
Joe