Lets keep in mind that this whole thing is a mental exercise - if the Pandora team want to use any of it, thats fine. For the rest of us, it's just an entertaining exercise.
From top down
case
screen
controls
controls
pcb
things on pcb
case
battery
case
There isn't a lot of room for thinning - except maybe removing the plastic between the battery and pcb
mmm.... I'm thinking that we could thin existing pieces using alternative construction - or move them to the edges. I see the materials order more like:
Case lid (thick for strength and screw holes)
Wifi antenna
LCD cable connector
LCD/speakers/hinge
case/bezel
top half of key tops and high rise gaming control tops
case & lower half of the key tops
keymat
motherboard top features
motherboard board itself
motherboard bottom features
case (thin)
battery
case (thick)
I do not see this getting as thin as a cell phone. They're into the sub-9mm range now (Galaxy S-II is 8.9mm). The Pandora as it is now is 27mm thick. Even with keys, 3x the thickness of a phone is a bit out there.
So - I think an objective of sub 20mm (more than 2 smartphones) thick should be possible - without loosing large portions of functionality. To do this, we will need to address each of the layers above.
Here are my thoughts - yours will likely vary, which is part of the entertainment portion of this.
Case lid - stamped from 30ga (0.25mm) sheet aluminum. Designed as an open side and bottom envelope that the screen slides into. bottom edge of sides have notches for screen stop. Bottom edge has piano hinge.
Wifi cable is removed from the lid. It gets placed in the middle plastics.
Screen (5") slides into case lid followed by a plastic 'screen stop' piece to keep the screen from sliding back out. Device X/Z dimensions would be defined by the screen dimensions + the thickness of the aluminum sheet wrapped around it's edges + thickness of an insulating layer/film between them.
Speakers are removed from the lid to be included in the middle plastics.
LCD cable feeds through a slot in the screen stop. It is exposed on the inside-case side of the hinge. there is no mc-twist through the middle of a hinge - it simply goes from the screen, through the screen-block, through a 2nd slot in the middle-case (key surround) (where there is room for some play for the slack needed) and onto the motherboard.
Hinge - fashioned as a piano hinge between the upper metal and the middle plastic case bits. 1.5mm rod through the center of the hinge is threaded on both ends. 3mm OD knurled nuts on both ends allow the user to tension/de-tension the case lid at any point in it's travel.
The bezel around the LCD is no longer plastic - it is the curled over edges of the lid aluminum. Take any common metal picture frame where the back slides in from the bottom as a reference.
Estimated total lid thickness @3mm.
Game control tops and key height reduced by >50%. Right now they rise about 3mm over the level of the keyboard surround by about 3mm. If we take 1mm off the thickness of the surround by making it metal, then allow the keys to rise 1.5mm over it - then we'll have saved ~2.5mm here. This retains travel distance - I'm talking about removing the majority of the gel-cap on top of the keys, etc...
Key surround - same .25mm aluminum as top. Cut-stamped to have control holes and beveled edges so it won't cut the fingers. It would wrap over the left, right and front edges and snap in place to the middle-plastic. Screws down to middle plastics.
Device back, sides, front one solid tray-like piece of plastic that the motherboard sits in and the rest of the pieces attach to - rises in the back to connect to the lid piano hinge. Motherboard would sit down in it from -above- which would mean that controls and key presses all ride on top of this bit. It will need to be ridged so it won't torque/twist. Speakers will live in the rise to the hinge. Antennas will reside in the rear edges. Two buttons (like AXBY ones) will live on each shoulder - not as tall, with dimples/recesses around them so they net out flush to the case.
Case bottom is, again, stamped aluminum screwed to the middle plastics. The battery door is on the right side behind a locking/pivoting door. Use a battery like the existing Pandora one - but put the contacts on the end instead of the face of it (saves 1mm). To change it, unlock and swing open the door on the right side, tilt the device and it comes out. Slide new battery in. Close and lock the gate/door.
Somewhere in how I'm picturing that in my mind, it looses nearly 1 cm in thickness. Using stamped sheet aluminum for the broad flat parts (top, key surround and bottom) shouldn't be overly expensive. Using cast plastics for the center motherboard support helps keep that one very complex part cheap. Moving the speakers and antennas to the mid-plastics and allowing a few mm of the LCD cable to be seen combined with the piano hinge and knurled nuts eliminates the hinge complexity - and removes the strain from the cable.
The above was off the top of my head. No rulers or micrometers were used in this example. The challenge is to figure out how thin it 'could' be without loosing functionality.
Yes, it's a bit of a radical design change, but that's the fun of this subject area.