In a press release Apple yesterday announced to drop the “non-disclosure agreement (NDA) for released iPhone software (..)”, because it “created too much of a burden on developers, authors and others interested in helping further the iPhone’s success (..)”. Developers are now allowed to publicly discuss matters of Apple’s iPhone SDK. Anyway, Apple added this does not apply to unreleased software. Unreleased software will remain under NDA until release.
John Timmer of ars technica took the time to write an introduction into developing iPhone apps.