Should XtremeMac's car charger for the iPhone not be quite classy enough for you, you might want to check out one of the 2point5 spec.dock custom car docks. Up until now they've been solely for the iPod - routing the music through your ICE speakers - using specially designed docks uniquely made to fit into different models of car (usually in the ashtray); however it turns out that, if you decline the iPhone's invitation to enter airplane mode after docking, you can use it as both PMP and speakerphone when a call comes in.
They're still working on compatibility lists and the number of models the dock is available for is limited right now, but 2point5 seem pretty open to custom work (they also do some impressive system installs, such as the Mac mini shown above) so if you fancy a more integrated setup and you can't afford a new BMW then give them a call.
spec.dock iPhone compatible [Tuner Tricks]
Dension iPod connector from about 1.5 years ago works perfectly fine with iPhone. Charges the phone (obviously), plays music, displays song info on headunit (BMW) and slowly mutes out and starts playing ringtone via stereo, then you answer with headset (jawbone), hangup – back to music, slowly fades in.
Perfect!
Monster FM cable only charges though, no sound.