Microsoft have announced a pair of new mice that are the first of their kind to have a dedicated Windows button. You can check out the mice on the official Windows Blog.
The Sculpt Comfort Mouse is the fist up and is a companion device to the Sculpt Comfort Keyboard that was recently announced by Microsoft. A touch sensitive strip by the thumb groove functions as a Windows key. The strip has a couple of other uses than taking you to the Start screen though.
If you swipe or slide it then you can cycle through Store apps in Windows 8, or it will bring up a list of your apps for easy access.
There’s also the Sculpt Mobile Mouse, which is an evolution of the Wireless Mobile Mouse 3500. The Windows Button on this mouse works fine with either Windows 7 or Windows 8.