I want to give iPhone development a shot, but my coding PC is way more powerful than the Mac I have (a Mac Mini). Is it possible with the new Unity 3.0 to develop game purely on PC and use Mac as a tool to compile and upload binaries to iPhone only?
As long as your input scheme can map easily between iOS and desktop/web it's not hard. But there are enough platform differences that you're going to want to do regular iOS builds to test. If you're going to use and of the standard assets you'll want to start the project on you Mac to pull in the iOS stuff.
That being said, your Mac Mini has more than enough horsepower to do an iOS game.