Full Bluetooth integration with your car is sooooo nice. I bought a new car after I got the iPhone and having hands-free phone and being able to play music over the radio is quite nice. The other nice thing about an integrated system is that the audio shuts off when you're taking or placing a call.
Without Bluetooth, Sync, or auxillary jacks the only other way I know of is via a little FM transmitter that plugs into your phone and transmits on an unused FM frequency. It does a pretty fair job if you're not in a big city where it might be hard to find an empty frequency. You can get these things at Best Buy and other such places.