since like 4000 applicants have been able to write and publish applications to the App store, how can someone write their own application and use it on their iphone? i'd be interested in learning the programming language if that were possible.
You have to have the SDK, be signed into a Developer Program ($99 or $299 a year), get a code-signing certificate, and have the beta firmware that will allow you to upload and test your applications on a physical iPhone. To get started, check out iPhone Dev Center