One important feature of the Xbox One is the ability to "pin" any two apps to the screen, letting you Skype while you play Madden or use Internet Explorer while you play Peggle 2. You can also watch TV while you game using this feature and the Xbox One's HDMI input, but according to Albert Penello, Microsoft's Sr. Director of Product Management and Planning, it doesn't stop there.
During a preview of the Xbox One's interface and functionality he said that anything with an HDMI output can be routed through the Xbox One and pinned to the screen. This includes Blu-Ray players, TIVO, and--yes--even the PlayStation 4. When asked specifically about Sony's system he laughed, but confirmed that there'll be nothing to stop someone from playing an Xbox One game like Ryse: Son of Rome on half the screen while someone else plays Killzone: Shadow Fall on the other half. Sure, it's a little impractical, but it also means you could use the Xbox One to Skype with someone while playing a PS4 game. Hell, it could also apply to the Wii U, or the original Xbox 360; that's one kind of backwards compatibility, we suppose.
The next-generation is going to be weird, you guys.