T9 predictive texting is used on many cell phones. You enter all of your text using 9 keys. When one tries to enter "cool" the word "book" could also come up since it uses the same "2265" keystrokes to enter. So some kids have started to use the word "book" as a laudatory descriptive with phrases like "man, straight up, that iPhone is totally book".
Which brings me to wonder why we often have such a fetishistic relationship with our devices. It seems to me that a successful piece of technology should be invisible or at least transparent. When you watch TV it quickly becomes transparent and you cease to notice the device at all. Instead you are immersed in the world that is being displayed. A phone is usually like that also. Unless you've got it perched on your shoulders and you're getting a kink in your neck or the sound or heat is hurting your ear, you don't really notice it. It remains transparent unless something goes wrong with it.
People go ga-ga over a new flashy device like the iPhone but the enthusiasm will be short-lived if its usability doesn't deliver. It will be a truly successful technology when you cease to notice it. Instead of thinking "wow I'm engaging with this really book device by using only my fingers" you should be thinking about the conversation you're having in a chat session or the information you're viewing on the screen, or the movie world that you've entered by launching a video.
Perhaps the reason that we get so excited is because we intuitively sense that the iPhone's combination of features and abilities will allow us to attain a greater transparency and ease of use. Like any successful technology it should have the fluency of a second language that arises when you no longer have to concentrate on the grammar and can begin to truly express yourself.
But the iPhone user won't require predictive texting. You'll be able to type on a full qwerty keyboard and as a result you probably won't discover any book new usages.