Now, that [mock-up] definitely looks a) bizarre b) impractical and c) not very Apple-esque. Honestly, with the built-in keyboard, having a chunky, physical slide-out keyboard makes zero sense. BUT -
Could the sliding component possibly indicate a second sliding screen? Think Nintendo DS-style - a secondary screen that could be used or hidden at one's leisure... It would contribute to multi-tasking, it would expand the amount of screen space usable, it would make a hell of a lot more sense as an explanation for the sliding mechanism than a physical keyboard...
What do you guys think? Is it possible that the iPhone 5 will carry a secondary slide-out screen?
Disclaimer: Obviously this is just speculation on my part; I'm not officially affiliated with Apple in any way.