I don't think the tri-fold is at all the same as the Sharp Zaurus line. They were bi-fold.

The twist screen isn't part of the tri-fold. The "tablet" functionality for the tri-fold comes from hyper-extending the hinge between the screen and keyboard, not by twisting the screen. You don't twist it, you just "keep opening it past 180 degrees", and then you fold section 3 to cover the keyboard. (section 3 never covers the screen, the screen gets covered by the keyboard)

And I don't think it necessarily makes for a heavier device. If you take the same parts that are in the N800, and just move some of them to different sections, it should work fine. Possible examples:

section 1) screen, chat camera, speakers, mic, headphone jack, and buttons*
section 2) thumb keyboard, batteries, and power plug
section 3) memory cards, USB Host and USB Client plugs, and picture camera

(* both face buttons and edge buttons, like zoom in, zoom out, full screen, and power, just like on the current design)

The only weight I've added is: hinges, thumb keyboard (should be very light, given the size and weight of the freedom input slim bluetooth keyboard), a second keyboard, a USB Host plug, and some extra plastic for the extra casing. Though, I bet you could add a second battery into section 2, and it wouldn't be a problem.

The question is: where do the logic boards for the CPU, internal memory, and wifi go? Maybe spread throughout all 3 sections. Dunno.