Apple have released an iPhone optimized User Guide for the new iPhone 2.0 firmware. It can be found in the iPhone Safari bookmarks or at help.apple.com/iphone.
The index page is broken down into iPhone functions or topics – Getting Started, Basics, Phone and so on. Each topic expands to reveal more options and information on the various functions of the iPhone.
The guide seems to be aimed at new users but I am sure there will be something for everyone when you start to digg.