I don't think keyboards which are actually large touchscreens will be particularly difficult to make from a technical viewpoint, but they will be a usability nightmare. Without tactile response, the fingers will start to hurt after a while.
There are more possible innovations in the direction of the Blackberry Storm's touchscreen (that depresses-and-clicks). Reports are it's a little uneven, but what if the resistance were perfectly tuned and consistent? That offers solid feedback about intentional keypresses while giving the fingers a place to rest.
There's still the issue of finding keys by touch. What if in combination with the above, a tiny amount of vibration could reward or discourage finger position, at the center or edges of active press regions? Or, longer-term, perhaps the screens themselves could develop subtle addressable rough spots and ridges as needed.
The last thing I saw that remotely resembled an interesting "concept design" was when Apple told a bunch of people they were making an iPod phone with multitouch, a full-featured web browser, cover flow, and on-the-fly Youtube access, but that wasn't a concept design, that was a product unveiling, because Apple doesn't do concept designs because Apple thinks they're bullshit as well.