iOS theming has come a very long way, and that’s all possible because of Dreamboard. There are plenty of great themes that complete change your iDevice’s user interface.
You can make it look like Android or Mac OS X Lion. It’s pretty incredible. To use Dreamboard, you just install the app and choose the theme you want. If you want to use a theme that doesn’t come pre-installed, just download it and copy it to the Dreamboard directory in the root of your iPhone. (You can use SSH or iExplorer to accomplish this.) Once you’re done, just open Dreamboard and select the theme that you want.
Dreamboard is known for overwriting lots of system files, though, so be sure to back up before you install!