Assuming you already know a bit of C and that you like learning by tutorial I'd recommend working through "Cocoa Programming for Mac OS X" 3rd Ed. by Aaron Hillegass. You will get a good tutorial-based introduction to Objective C, Xcode/Interface Builder and the design patterns used to build both OSX and iPhone apps. (iPhone is not mentioned in the book at all but the general patterns are the same.)

Be sure to understand the retain/release memory model as the garbage collector is not avail. on the iPhone.

With the background you gain from Hillegass you'll find it very easy to pick up the differences between OSX development (which uses AppKit) and the iPhone development (which uses UIKit). There is a lot of good information found at the iPhone Dev. Program's website which bridges the gap.

The tutorial approach is always a little frustrating to me personally as often details are glossed over and the topics are not covered systematically but if you supplement the tutorial with the information provided by Apple you can resolve all the little annoyances as they come up.

Finally I recommend watching the videos Apple's prepared about iPhone development.

Good luck and have fun!