So you want to be an iPhone developer? Great! But first you'll need some stuff:
1. An Intel based Mac. A MacMini fits the bill. The MacMini is the cheapest way to join the Mac universe, and takes up little space. Desktops not your thing? The Macbook White is the cheapest laptop Apple makes.
2. A membership in ADC (Apple Developer Connection), which is easy enough to get if you have an iTunes account. If you want to test your app on your iPhone and distribute it in the app store, you'll also need $99 and apply to the iPhone Developer Program.
3. The iPhone SDK, now available at www.apple.com. The current SDK is for the iPhone 2.2.1 update.
4. An iPhone. I suppose you could just test your programs on the simulator, but what fun would that be?
5. OS X Leopard 10.5.5 If you're still using Tiger, this is your justification to upgrade. Plus it has Time Machine, Spaces, and some other cool features.
Good luck on your development and I hope to see your app in the App Store soon!