You could perhaps have a layout:
1 2 3 sync requ send
4 5 6
7 8 9 up
H 0 . ok can down
dddd = the display
H = home icon, this would also wake up the device out of a sleep mode. (long press to switch off + you would have a 5 minute auto-sleep)
ok = ok, confirm payment. A green tick icon. Big key.
can = cancel. A red cross icon. Big key.
sync = sync icon
requ = request payment icon
send = send payment icon
up = up arrow
down = down arrow
By having it all icons the keyboard is fully internationalised. You would set the language of the display in your sync software on your PC.
You would put a border around the separate functional key groups to 'join them up' and to square up the keyboard layout.
If you were using infrared you would want the LED on the top edge of the device, in the middle. You would have a little alignment mark on the top of the case That way when transferring BTC device to device via infrared you would have your device pointing at the recipient's and both could work the two keypads simultaneously.
You would put the microUSB connector on the back edge or the right hand side at the back. You want it at the back so that the device is wedge shaped. Thin at front, thicker at back. That makes the keypad slant towards you when it is sat on a table top. You want it on the right as Apple USB ports are typically on the left of their machines so the cable connection works better.
The position of the LED and microUSB suggests the batteries go at the back left, accessible through a little slot cover on the underside of the device. Circuitry is probably across the centre of the device, leaving only the membrane keypad at the front so it could be thin thin thin at the front edge.
You would also want a Bitcoin logo somewhere. Perhaps to the left of the screen as then you could have the balance (say, 12.345 BTC) right next to it and the two would be visually associated.
It would be thicker than a credit card (because of the microUSB connector) but I wonder if it could be the width and height of a credit card ?
Then it would be easier to fit into existing (physical) wallets people carry.
Anybody any good with photoshop ? :-)