The iPhone OS SDK is very similar to the SDK for Mac OS X. It is in no way drag and drop or ‘quick build’ if that’s what you’re under the impression of, you will need to learn about interfacing with Cocoa and the associated iPhone APIs, and a knowledge of the programming language Objective-C will be required.
To get an idea of how hard this will be, you could have a quick browse of a beginners’ Objective-C tutorial (http://cocoadevcentral.com/articles/000082.php) and Cocoa Dev Central (http://cocoadevcentral.com/) will help you to learn about the general development process itself.
Apple’s Developer Connection (http://developer.apple.com) is useful for finding out API information once you have a good understanding of Objective-C.
A friend of mine—a Mac developer who makes his living off app development—says it took him around a year and a half before he had a good understanding of the dev process and was not continually going back to reference books and manuals.