Developing on the toolchain

[!ein!]

I've jailbroken my iPhone and am really itching to start developing apps for it, but I can't quite find sufficient documentation on how to do this. I've got the toolchain working and got the HelloWorld app to work, but that's about it. Can someone point me to some good resources for writing iPhone apps? Is becoming an apple developer the only way to get access to documentation? I saw http://www.cocoadev.com/, but it doesn't quite have enough details to be very useful.

Thanks for your help!