The jailbreaking community has brought all sorts of interesting tweaks and hacks to iOS devices. Lion Ultimatum is functional theme for jailbroken iPhones that attempts to completely replicate the OS X user interface, not just with icons and a background, but with "fully usable Finder menus, stacks, launchpad, a scrollable dock, draggable windows, mission control, dashboard, and a customizable lock screen."
Lion Ultimatum is a free install for jailbroken iPhones. Lifehacker has installation instructions for those interested in trying it out.
It is, however, more useful as a proof-of-concept than anything else, and shows why Apple redesigned the iOS user interface from the ground up to be touch-compatible, rather than simply shoehorning the OS X interface onto a smaller touchscreen.