I spent a couple hours yesterday putting this together. This is my new desktop environment for my G5 Mac, running OSX 10.5.

The background image is a mockup of the Wii's home menu, centered in the upper-right corner of the screen. The icons within the windows of the menu are functional buttons that fade in and out when you mouse over them. The "Settings" button opens the System Preferences, while the "Exit" button puts the computer into sleep mode (or shuts it down, but I usually just use sleep). The buttons were made through creative use of Yahoo Widgets.

The chat program is AdiumX, with a Wii buddy list style and sound set (using system sounds ripped right from the console).

I copied this same style to my PC laptop, including the same functional buttons, but I had to put in additional ones for specific programs that aren't on the Mac desktop (since I have most programs linked on my dock). Also, I couldn't find an equivalent Wii-themed skin for Trillian, so the look is a bit different on the PC, unfortunately.

Official links to stuff used in this desktop:

Yahoo Widgets - The core program needed to run the widget system. Both Mac and PC compatible.
Animated Shortcut - The specific widget used for the buttons.
AdiumX - A Mac-specific chat program. The best one you can find, and most easily customizable.
Adium Wii Contact List Theme
Adium Wii Sound Set
Adium "Candybars" Message Style - Use the "Blue Vs. Gray" variant to match the Wii's color scheme.

The icons for the buttons themselves are just transparent .PNG files put together in Photoshop. They shouldn't take you too long. If you want the original hi-res version of this desktop (in 1680x1050 resolution), click here.

Now all I need to do is figure out how to get a Wii Remote to work with MacOS X.