Rearranging the icons on the home screen of your iPhone is a part of the customization fun that Apple built in. However, if you rearranged your icons and want them back to factory specifications, it is as easy as a simple tap. Just navigate to Settings > General > Reset. Once there, select the "Reset Home Screen Layout" button. A dialog will pop-up asking for a confirmation.
Once you return to the home screen, you will notice that all the icons are placed just like they were when you first turned on your iPhone! Third-party applications' icons will follow the default applications.
Want more iPhone tips and tricks like this? Just visit the TUAW iPhone 101 section.