Adding a desktop button to the Dock or Menu Bar
While you could create an AppleScript application that triggers Expose's reveal Desktop function, save it, and put it in the Dock, an easier solution would be to use Hot Corners. Then, when you put your mouse in a screen corner, it reveals the desktop.

If you wanted to go the AppleScript route, you'd create a script that said tell application "System Events" to key code 103, save it as an application, and put that application in the Dock.



To go the hot corners route, in Lion, the preferences is under Mission Control in System Preferences.



While it is certainly feasible to create a program that puts an icon in the menu bar to reveal the Desktop, it requires programming techniques beyond the scope of this site.