This program can no longer be downloaded. You can look for alternatives in Apps > Customize Your Mobile > Wallpapers & Themes.
W Phone 8 is a Windows Phone 7 app that simulates the appearance of the Windows Phone 8 user interface.
Run W Phone 8 and you'll be presented with a screen that mirrors the design of the Windows Phone 8 home screen. You'll notice that live tiles come in a series of different sizes and icons can be resized and rearranged at will. The color scheme of the tiles in W Phone 8 will be based on the theme you've already selected from your device's settings.
W Phone 8 looks neat and makes for an interesting preview of Windows Phone 8. However, it's functionality is very limited and you won't actually be able to do much with it. For starters, it won't set itself as your default start screen, it will only appear when you load the app.
What's more, most of the apps that appear as tiles on the W Phone 8 interface aren't useful. Most just link to the apps' page on the Marketplace (whether you already have that app installed or not), and some (the Calendar, for example), don't do anything.
If you'd like a preview of what Windows Phone 8 might feel like then try W Phone 8. However, you'll probably want to uninstall it once you've played with it for a few minutes.