It's downright irresponsible to recommend such things in a book on developing for the official SDK. The people reading the book are likely people who don't understand the system well enough to safely use private/unpublished APIs like Gruber outlines.
That said, Apple should open up most of these APIs (CoverFlow, prox sensor, etc).
"Some of the nicest bits of iPhone programming are included in the public iPhone frameworks but not in the SDK. Apple's unofficial policy on this is clear: You can use these items in your programs, but you do so at your own risk. Your code may break at each firmware release. Striking the balance between risk and reward if up to you."
edit: I guess I'm still getting the hang of HN etiquette. I was impressed by the clint's usage of an unusual word and got downmodded for expressing that concisely. So, next time do I use a complete sentence to express my admiration, or do I avoid any discussion of word choice / writing style?