Same as the Nokia N900. A querty on a slide under a screen exchanges a little thickness for smallness (you can make a screen smaller if the keyboard isn't always taking up half of it.) As a bonue, you get a trustworthy, fast input device.
The Nokia N900 is simultaneously I really wish I had gotten my hands on when it was new and also a device I'm glad I ultimately didn't wast my money on. The landscape slider is still a form factor I miss though. Sure I've mostly adapted to onscreen keyboards, but I occasionally still yearn for something physical.