Arrange Icons on the iPhone/iPod touch Home Screens
Unhappy with the arrangement of your icons? You can move them around as follows: First, hold down on any Home screen icon until all the icons wiggle. Now, drag the icons to their desired locations (drag left or right to get to other screens). Finally, press the physical Home button on your device. (Unlike earlier releases, iPhone Software 2.1 doesn't move just-updated apps to the end of your Home screens, so your icons should be more stationary once you've installed the update.)
Remember that you can replace Apple's default icons in the four persistent spots at the bottom of the screen with your four most-used apps!