Apple's new iPad apps—iBooks, Pages, Keynotes and Numbers—wouldn't be accepted into the App Store if they were submitted by an outside developer. That's because they use private APIs that other devs don't have access to. Yet.
Some features that these first party apps have, such as access to the system dictionary or true brightness control, aren't available in the current iPad API. This frustrates developers like Marco Arment, who created the popular Instapaper app.