Folklore.org has the full story of how Steve Jobs convinced Bill Atkinson to give the Mac OS rounded rectangles as they were developing the first versions. I won't ruin the story, but if you played with early versions of Windows you may remember it had sharp corners, not rounded ones. Rounded rectangles have been a design concept at Apple for a very long time for a reason.
So it was largely unsurprising to me that Apple just received a design patent for rounded rectangles -- although these are for hardware, not software. It is specifically for the iPad, in fact.
Ars has the full breakdown of patents surrounding this one, but it should be noted that design patents are very narrow. Someone would have to quite blatantly rip off the iPad's design to infringe and they'd have to apply the design to a tablet device like the iPad. Plus, design patents do not touch functionality, and are specific to the object the design is for. As an Ars commenter notes, George Lucas has a design patent on Yoda. It's pretty standard procedure and is unlikely to affect any current legal wranglings in the courts.