We're not sure if this is true, but Yasuhiro Yamanaka of Sony Computer Entertainment claims that he "designed PSP and PS3 User Interface all by myself." You can blame him for everything.
In a regular company, it would be very difficult to believe that one man designed the entirety of any UI, especially one as large, complex and important as the PlayStation 3. But we're talking about Sony here. They're the people who designed the hardware for the PS3 first, then dumped the components onto the software guy, saying "do something with it."
If you're like Mark (and many others), you'll congratulate Mr. Yasuhiro Yamanaka on a job well done. If he really did create all of that by himself, it's quite an accomplishment. If you're like me, and think the XMB is pretty, yet functionally horrible from a UI development standpoint, you'll think this guy should hire a few more people to help. [LinkedIn via Superannuation via Joystiq]