Apple has published plenty of code samples for the iPhone SDK. They are very instructive, but unfortunately they are not very consistent in style and use of Cocoa. It’s hard to tell what the best practices for iPhone programming really are.
Meanwhile I came across Cocoa Dev Central, an excellent web site full of Cocoa tutorials, articles and information. The site is not focused on the iPhone, but there is plenty of good information for budding iPhone programmers. I highly recommend the Objective-C Style I and II tutorials.