The problem, of course, is that there's no single device, cable or solution that will manage calls, music, and charge your iPhone's battery simultaneously. I'm using a set-up similar to Jeremy's budget solution -- a cheap mount holds the iPhone in place, an audio cable connects it to my stereo's audio in port and a Belkin charger keeps the battery going.
Of course, I've still got to pick the thing up and disconnect the audio cable to place or receive a call (still legal where I live). I know that a Bluetooth headset would eliminate that issue, but I don't want to have to buy one more thing to accessorize my iPhone.
Jeremy's "Optimal Solution" eliminates a lot of the stuff with a Contour Design / Bluetrek SurfaceSound Compact, which is a nifty hands-free solution for mobile phones (though quite pricey). If price is not a concern, check it out. As for the rest of us, have you got your iPhone set up in your car? Share your solutions below.