During the night, you connect up your MP3 playing Mobile Phone 'Face' to your PC, and download/Podcast your music content for the next day.
In the morning, you unplug your 'face', get into your car, and re-attach the 'face' to your car stereo. You begin to play music straight away, whilst in the
background, several things are happening...
a. The MP3 playing 'Face'/phone is charged up.
b. The tracks on your 'Face' are copied onto the Car Stereo's HD, for later play.
c. You can also copy music from your HD onto your 'face' as you drive/play.
d. Hands free functionality.
e.The HD on the Car Stereo could also store other content playable by your phone, like video, photos etc.
Then when you get to your destination, you have an MP3 player/phone, fully charged, with a collection of tunes. You also you only have 1 gadgety thing instead of 3 (MP3, Face, Mobile.) in your pocket.
The Car Stereo is also a radio/CD player, with MP3 Encoding - if you like something you are listening to at that point, press the big 'Record' button, and it goes either onto the HD, or onto the MP3 playing Face/Phone. Similar functionality as car stereos that record traffic news, but in 192kb.
Edit note:
So that you can change mobile phones and not have to change car stereos (duh me), this might be best achieved using a docking cradle in your car with an 'adaptor' insert to make it compatible with many phones. You could then use the Bluetooth 'Passkey/pairing' concept to allow connection with all phones it is paired with. Your Car stereo would be supplied with a passkey, and you simply pair up any phone you want to use as your 'face'. The car stereo would only work when it was paired with one of the supported phones.
You could actualy use just bluetooth for the whole thing, and not need a docking cradle, but there might be security issues I guess. Also, you'd lose the benefit of in-car phone charging.