There are a lot of very good reasons why Apple doesn't let you to skin the iPhone's UI (just glance at most BlackBerry or WinMo themes). But then there's this awesome retro Megaman theme.
It's like an 8-bit Capcom fanboy wet dream that's congealed into a custom iPhone theme. Sure, like most of the apps populating that App Store, the vast majority of themes would probably terrorize your eyeballs in ways that would make Gitmo detainees cry. But! A few gems, like this one, would undoubtedly peek out of the mountain of shit.
Apple's attempt to protect you be damned, if you want it really bad, here's how to jailbreak your iPhone so you can put this (or whatever other unseemly graphical user interfaces you want) on your phone. [Mac Themes]