Apple's iPhone Dev Center is the place to go to become an iPhone developer.
Apple's iPhone Developer Forums are available to registered iPhone developers.

MacRumors iPhone Programming Forum
iPhone Dev SDK Forum
Stack Overflow is a q&a site for developers. Chances are good your question has already been answered here - and if not, you can post a question and get an answer fast.

Ray Wenderlich has lots of great tutorials for iOS development.
ManiacDev.com posts regular iOS Development tutorials and programming tips.
iPhone Development Blog by Jeff LaMarche, author of the Beginning iPhone Development book.
Cocoa with Love - Matt Gallagher's Cocoa/iPhone programming blog, with example projects.
Theocacao - Scott Stevenson's Cocoa blog.
iPhone Developer Tips by John Muchow - more useful code snippets.
NSBlog by Mike Ash. His Friday Q&A postings cover some more technical C and Objective-C topics.

Conferences for iPhone Developers