Given how many people come here whining about keyboards, I should think there is a humongous market for it.

Personally: Nèh... It would make it too bulky and/or quite unfit for the three main tasks I use it for: ebookreading, movie watching and music listening.

Of course, if we had a proper word processor and database manager, a built-in keyboard would make a whole lot more sense, wouldn't it?