Have you ever wished your iPhone displayed Safari in portrait mode while holding your iPhone horizontally, e.g., when lying down on either side of your body. Well, read on. In order to lock your iPhone in portrait mode, all you have to do is exit to home screen, turn the iPhone horizontally and finally enter Safari (or any app for that matter) and voilà! Your iPhone is displaying Safari in portrait mode while you're holding it horizontally.

Edit: here are some photos to back up my finding:
Now I challenge you to reproduce all of the above in 2.2.1 and 3.0. See, doesn't work. Because it's a new feature in 3.1b.
Edit: here are some photos to back up my finding:
Now I challenge you to reproduce all of the above in 2.2.1 and 3.0. See, doesn't work. Because it's a new feature in 3.1b.