Everyone knows how Apple "stole" the GUI from Xerox PARC (they actually licensed it fair and square, and lucky for us, because it's unlikely Xerox would have taken it to the mass market if Apple hadn't) and then Microsoft "stole" it from Apple (though Microsoft licensed GUI tech from both Xerox and Apple). After the release of Windows 2.0, Apple sued Microsoft because they believed that the license agreement only covered Windows 1.0, but Microsoft prevailed in court. It's an interesting story you can read more about here. And of course, Linux desktop tech is notoriously derivative of existing GUIs. Whenever someone develops a user interface that deviates drastically from the desktops we know, it fails to catch on, because, like it or not, people like familiarity. So we've seen an evolution rather than a revolution, and during that evolution, there's been plenty of trading back and forth. And that's been good for everybody.
Apple no longer enjoys the lead in GUI elegance that it had in the Windows 3.1 days, of course, and some might say that Apple has made major mis-steps in the past decade. Microsoft seems to have made steadier progress, as each Windows release has been markedly better in usability. But we now have a choice between several very usable OSes. It's a little boring. Of course, we're now facing a brand new personal computer land grab: mobile. In this one, it's 1993 all over again, with Microsoft stuck with a workaday mobile OS and Apple pushing the envelope. This time, there are a few more viable OS contenders, with Nokia, Google, Palm, Intel, and others all scrambling for a piece. No doubt the "stealing" will continue, with each new mobile OS release emulating the best parts of the others. Let's hope so.