You could keep the old iphone or an old ipod just for the GTi? I'm pretty sure you can sync two ipods to one copy of itunes so you could just take it out of the car occasionally to sync it?
Thats what I was going to do when I upgrade to an iphone 
Yeah that would be a good idea, however, I found out last night (for anybody who is interested), that if you remove the tray in the armrest that houses the dock at the bottom, for some reason, you can then plug your 3G into it (while its removed) and then slot the whole thing back into the armrest, and it fits & works!
To remove, just put a flat ended screwdriver down the back of the the section you slot your iPod into, which will loosen the two glue spots that hold it in place, then you can pull that section out (the dock is not 'plugged' in, it connects into a circuit board via a few gold prongs). It takes a bit of getting out the first time, but once its out and loose it comes out and goes back in as easy as your iPod. Then for some reason the 3G fits in.
