Some will say Micorsoft took their GUI design from the Macintosh. Some will say Apple took it from the XEROX Star. But it goes without saying that there is very little distinction between today's Operating System GUIs. It appears the future may not be any different. Micorsoft's up and coming Longhorn “next-generation” GUI seems to incorporate some very OS X-like interface features.
"Another new feature in Longhorn is a vertical panel on the side of the screen that could include essential information at a glance, such as the time and date, links to favored Web sites or updated stock prices." Sounds like Apple's Dock to me. Longhorn allows you to "stack" documents, which sounds a lot like Apple's rumored Piles.
Even Sun's Looking Glass 3D UI looks a lot like a modern desktop GUI with a little bit of added depth.