Haptic feedback on a smartphone touch screen

Over the last few weeks, I've had the chance to test a few devices that utilize various forms of haptic feedback when you're touching the screen to input data.

• The BlackBerry Storm uses a "clickable" screen that acts like a button (SurePress technology). You have to press down on the screen like a large button to enter most of your data. This way, you're "sure" when you're entering data.
  
• Several Windows Mobile devices use a tiny buzz/vibration each time you tap on the screen. This is something that is controlled via software, so you can customize it quite a bit. This feature is currently available on the Samsung Omnia and also on several HTC phones (through custom ROMs and TouchResponse over at XDA developers).
  

The Apple iPhone does not come with any haptic feedback. This feedback is probably most useful when you're typing on the screen. My guess is that future touch-screen devices will all incorporate some type of haptic feedback so that users can achieve a higher level of precision and efficiency when they type on the screen. I think it would be very fascinating to perform a study to evaluate the effects of haptic feedback on a touch screen. Would it improve accuracy and efficiency by 5%? 10%?