Microsoft’s Metro UI owns the square. Apple has a corner on the roundrect, from the Springboard launcher to the iPhone hardware itself. Nokia, despite its late entry with MeeGo’s Harmattan UI, found the squircle unclaimed and ran with it beautifully. Palm has used the circle from the early days of PalmOS, and in WebOS, HP continues the tradition with care (one might even note that both Palm and HP structure their wordmarks around the circle).
I have a nagging feeling this observation will become useful on future projects. (Via John Gruber)