If you want to customize the iPhone theme then you can use online iPhone theme generator but for personalizing and creating the theme including presentation, layout, and the smooth integration with third-party applications for your Apple iPhone, iThemeIT is an excellent option.
Here’s what you can do with this application:
Update July 25th, 2012: iThemeIT is no longer available. The website redirects to iPhone forum, and unfortunately the tool is dead. Please check out these beautiful iPhone wallpapers available for download.
Download iThemeIT for iPhone [Link Dead]