iPhone SDK can be downloaded from Apple now. There is a free version you can test and play with. It is $99 to become a registered developer and get a key so you can create applications people will be able to download from the iPhone application store beginning in June. For $299 you can obtain an enterprise license which allows you to develop in-house applications.
More details on the 30%Apple/70% developer split and tons of examples, demos, how tos etc are available on the Apple developer site.