There's been a lot said about MEMS (microelectromechanical systems) lately, those devices that can sense direction (or change in direction). Specifically, an article in EE Times quoted Phillippe Kahn (yes, that Phillippe Kahn, of Borland fame) as saying that there will be 10 billion MEMS chips in mobile phone by 2010.
There are many reasons why you would want these devices embedded in your handset, many more than I can think of. First, as evidenced by the iPhone, the MEMS device is used to determined whether the phone is being held in portrait or landscape mode (up and down, or sideways). That way, the image is always displaying in the proper orientation.
A second application, one that I've already seen implemented, is to serve as a way to page through an application, such as a phone book. Rather than repeatedly hitting a button to page through the phone listings, you simply snap the phone forward, as if you were sorting through actual pages.
If you're a gamer, the MEMS parts could really come in handy. Rather than hitting buttons to play the game, you could simply rotate the handset. Want to hear the next song in the playlist? Give a shake. Want to answer the phone? Hold it up to your ear. And the list could go on for a long time.
Ten billion might be a stretch, but not necessarily by a long shot. If you assume that some large percentage of handsets contain three MEMS (one for each axis), and around one billion handsets are shipping per year, I could see a few billion of these devices shipping.