My previous Musicbox system has now been replaced by my iPhone. The new setup is superior for many reasons, including the far nicer screen, the elimination of the boot-up time and of all the extra crap in my car, and the fact that the iPhone is always up to date with my newest albums and podcasts -- not to mention that it has all my photos, email and web access, maps with turn-by-turn driving directions, etc.
The mount is simply a small block of wood with a spot routed out for the iPhone dock to sit in. The dock's USB cable is plugged in to the car's 12v power outlet using the InCharge Auto, and its 1/8" audio output is going to my amps in the car's hatch. I also had to add an inline volume control (Radio Shack #271-1732) because the audio signal coming out of the dock is a full-blast line-level signal that's unaffected by the volume control on the iPhone itself. The silver knob is one of the many sweet retro knobs that Radio Shack carries.
The dock is secured to the wood via a piece of velcro on its bottom, and the wood is secured into the car's removable empty-head-unit-slot thing via some extra-strong velcro-like stuff that I got at Radio Shack -- it's called "Superlock Fasteners" and it's part number 64-2360.
I need to drill the volume-control's hole a little deeper, so it sits flush with the top. To finish it off, I haven't decided whether I want to round off the edges or just cover the whole thing in black felt.