iSwitcher is a theme swapping iPhone application and is 100% compatible with SummerBoard (verified with all versions up to 2.1). This app allows on the fly theme swapping. The theme file is stored as a (ditto) zip file in /var/root/Media/Themes. iSwitcher extracts the zip file and then for whatever apps you have installed and that you have icons for, it installs new icons. There are also some default theme packs and icons to get you started.

Use iPhone mods and hacks at your own risk! Installing 3rd Party iPhone apps requires you to jailbreak the iPhone before installing the native software and apps. You can use Installer.app for Mac OSX and iBrickr for Windows PC to do this. Browse the full list of all iPhone and iPod touch native 3rd party apps filed under the software category.

Source code here Get it via iBrickr or Installer.app