I'm interested in developing apps for the iPhone, but mostly third-party. What will I need to do this? What language do I need to learn? Where can I find everything I need to do this? If I decide to publish the app through the Apple Store, I'm aware that I have to register as a developer and pay $99 for the SDK as well as a key correct?

Can I use another package to develop apps without paying the $99 for the SDK? Also, is there a way to develop on a PC running Windows XP Pro? Buying an Apple computer is not possible for me right now.

Thanks for your help and suggestions!