Just a second thought on your post:
“There are some limitations to Ghost Fingers typing: namely that you could never lay the device down flat to type; unless you have a USB/bluetooth keyboard or a more traditional touchscreen keyboard, you’d ”

Off course you can use a regular touch keyboard instead, but it might be a bit confusing to user to switch from one layout to another….
You might introduce another pattern a new?!? pattern I call the “Permeating Fingers Pattern” to avoid this drawback:
1: your Tablet is put flat on a table
2: on the screen is displayed your overlayed keyboard left and right
3: On the bottom of the touch screen you have a special zone which transform your touches in a second layer of interactivity (a bit like control/command key pressed + mouse click).
When you you click the zone –> activate the feature.
Activating the feature might be to hold button pressed as Shift key, but I feel it would be better to have a “Caps Lock” approach to allow both hands to be free.
Once the “Permeating” feature is activated,
it allows to directly interact with the backscreen representation of the keyboard. A bit as if physically your fingers can go through the screen and tap the back of the case.

This “Permeating” feature might be activated also automatically through gyroscope or special routines in the back case sensitive zone.

Your back keyboard idea brings also new problematics to address, like how do you communicate in the interface representation that it is not a regular touch interface but one that is managed by interacting with the back of the case of the device?
A way might be to use a “revert colors filter” + blur approach?!?

I could have some time to test some mockups, do you mind to send me a vectorial version of your keys design?

Thank you in advance

“Less of the screen would be obscured, you’re instantly shown if your fingers are in the correct position and the shape of the overlay would change (pattern & motion recognition) as you naturally move to type.”

I vote for that an on screen translucent representation of the back keys. By the way it needn’t to be 1:1 scale representation.

Might be a 50% scale representation (like in your example) with hovered keys being emphasized by a bigger size?!?
This would allow also to match user experience on using this back keyboard. The more the user is used to it, the smaller the transparent representation of back keyboard might be and the less obstructive it is to displayed content. It might be a great “adaptability to user knowledge” pattern…

By the way the upcoming Notion Ink Adam tablet has a back track pad panel which might mean that we aren’t that far from your suggestion.
http://www.youtube.com/watch?v=eQvefuCHZds

In addition I read that Apple registered a patent for a “back-side touch”.
http://bit.ly/bO1Axf

An some rumors:
“Next-Generation iPhone to Offer Touch-Sensitive Casing?”
http://bit.ly/bO1Axf

Best regards.

Cedric Madelaine (aka maddec)
analogd interactive
http://analogd.com
Twitter: maddec303

Cool concept. You could take the Ghost Finger pattern a step further in your concept and consider using a translucent overlay on the screen that shows the virtual keyboard on the screen as it is laid out on the back (the mirrored image of the iPad mockup). It would allow users to essentially punch the keys “through” the screen. This would likely reduce the time to learn the keyboard layout (recognition instead of recall). It could be configurable, so expert users can turn it off.

Looking forward to reading about the next steps.

-Kris