If I'm holding the phone in landscape with thumbs on front and fingers on the back, what I'd really like is buttons on the back so I'm not limited to just my thumbs. Shift, spacebar and an 'alternate layout' button (switches the onscreen keyboard to symbols/accents) come to mind. Maybe navigation buttons so you don't need to constantly shift your hand to reach the hardware buttons.

That said, many flip phones do actually have dual screens. Never seen them used simultaneously though. (Woz's solution for multitasking also comes to mind - just have two phones.)

Seems easier to long-press the home key than to lock my current screen, turn the phone over, and unlock that screen. I don't think you've really thought this through...

Mightn't that be a bit of a UX nightmare when you factor in a touchscreen? Yes, you could use orientation to decide to only take input from the 'topmost' screen, but you'll regret that when you're trying to set an alarm/check email while lying back in bed. Or, put another way: you know how annoying it is when auto-rotate doesn't get it right? Double that.

That way you could be talking to a friend while you gchat a friend, and facebook another friend. Summary: Multi-task while you multi-task.

weight, my friend.