uiGarden.net has this interesting Usability Report on the iPhone.

Apparently texting on your Blackberry (or other device with physical keys for the letters) is more accurate.

Favorite quote:

On feature of the popup keyboard on the iPhone is the drag and lift feature which is said to reduce errors. Unfortunately not one user discovered this feature.

uiGarden.net post

Related evaluation of the iPhone against two other devices

UserCentric, the folks who did the usability testing