if you have aux in you just need a male to male 3.5mm jack lead to plug from the ipod headphone socket into aux in. This will allow you to play your ipod through your car, but it's controlled via the ipod.
Alternatively if your stereo does it you can probably get a special lead to enable you plug in via the dock connector giving you controls through the steroe. However, if it's a standard radio i'd suggest this is probably impossible.
Next stop is just buying a new radio, i know the Alpine IAX300 works, as my friend just got one in his alfa, looks nice and works with ipod and iphone... doesn't charge the iphone tho. It's also blue tooth ready so if you buy an extra 'box' for it you can take calls on your iphone via the stereo.
hope that makes sense.. basically you can do it now, but it's not an integrated system and to go intergrated, it's likely that a new stereo will be required.
-Cyph