iPhones take maximum 1A. If you use a switching DCDC adaptor to get down to 5v (like most of the tiny ones are) count on them being 75%+ efficient, which means you'll be taking at the most:

(5*1)/0.75/12 = 0.55A at 12v.

I believe the empeg might manage that, but I can't remember for sure. You'd be safer using a relay, really.

There was a desktop dock for the original iPhone that fitted the phone and the apple bluetooth headset, AND had audio out. iPhone 4's (and 4S) fit this just fine, and I may even have a spare one...