Hmm.. does the most recent Windows 2003 and/or the prototype Vista or even further away prototypes have a GUI much different from XP? I admit I haven't seen much of OSX, but the type of GUIs we have today have been around for a while and previous more obvious lawsuits from Apple have failed, so I can't see why another one would be more successful.
Besides, Apple should've been sued by Xerox in the 80'ties, because it was from Palo Alto that Steve Jobs "borrowed" most of the ideas to Macintosh first graphical environment.