If you're going to use the stereo (or ashtray) area to mount, then it's actually pretty easy. The S3 is a bit big for the ashtray area without cutting, but it's also thin enough that you could easily rig the mount in front and it won't protrude much. An iPhone 4s could probably be recessed into the dash (which is why I asked)
Wiring for it is actually really simple, depending on what you're connecting it to.
If your stereo has a rear USB, then pull a cable through the bottom of the ashtray mount and you're done. If not, then I'd probably go buy a power socket at your local cheap auto parts place and a USB adapter, then tap that into the accessory power at the stereo. Mount the power socket in the ashtray space, and get a low profile USB charger adapter for it. If it's front USB only that's harder, but if the stereo is connecting via bluetooth, then it's a done deal again.
To mount the phone, I'd look for a cheap
eBay case for the S3, and mount that to rails that go in the ashtray mount. Put some sort of backing on that to make it look decent when the phone isn't in the car, and probably make it so that I could change the case when I upgrade to whatever the next phone is. Maybe set it up so that you put small screws through the back of a hardback case into a finish bezel. Change to a different case, drill a couple of holes and screw it on. Chances are that the micro-USB charger/connector is going to stick around for a while, but even if not, if you make the power wiring simple USB 5V, then it's practically guaranteed to be useable for whatever phones come out in the future.