Microsoft Research is working on a slate/tablet-related projects "RearType," including a way to add physical keys to the backs of these kinds of mobile-computing devices. Researchers have taken two halves of a QWERTY keyboard and rotated the keys in a way that user's thumbs remain on front of the surface and other keys are placed within reach of the thumbs. An on/off button for activating/deactivating the keys to avoid accidental typing is a must, the researchers note.
RearType paper describes the project: "Our goal is a system that provides the tactile feedback and familiarity of a regular keyboard without cluttering the front of the display, ameliorates the occlusion problem inherent in direct on-screen touch and pen input, doesn't use the valuable screen real-estate taken up by an on-screen keyboard, leverages users existing skills in touch-typing on a regular physical QWERTY keyboard, and allows for text entry in highly mobile usage scenarios."
Reference: Research Paper