Although I have been an Apple developer for a while I haven't done anything as far as learning how to code for OSX and I am not an iPhone Developer. Becoming an iPhone developer is actually an easy process. Just sign into your ADC account a sign up.
There are 3 options for membership when signing up to become an iPhone developer. Two are paid and one is free. To start with I have gone with the free option though it seems there are some limits. (write down the limits).
I also downloaded the special content from Apple for iPhone developers. Really it's just a series of videos that take you through Xcode, Dashcode, and Cocoa (the native OSX programming language).
Finally to actually be able to code for OSX and iPhone you need to download the iPhone SDK which includes all of the applications for writing code.
Unfortunately since I am not a paying iPhone developer I don't have access to the iPhone 3.0 SDK. I haven't found any info on when this may become available to non-paying developers but I hope it's soon. The only application that I can think of building that isn't out there wouldn't require the new SDK but the function would be enhanced by push notifications. Ah well maybe I'll pay someday.