This is probably true for companies targeting casual users. Facebook and YouTube are great examples. But I can't see people switching to mobile for the bulk of their serious, work-oriented computing unless the hardware experience drastically changes. I would never program on an iPhone or iPad if a computer were available. Nor would I want to do serious video editing, or CAD. I probably wouldn't even want to type a bo…
That doesn't mean you can't use a mouse and keyboard as well. And that does make it similar to the form-factor of a laptop or a desktop but that misses the core advantages of tablet computers. The OS experience, the app. install and management experience, the portability advantages (tablets are vastly more portable than even a laptop), and the potential cost savings. Not to mention that it could easily be a different set of companies providing such systems in the future than the PC stalwarts of today (perhaps google, apple, samsung, and HTC instead of Microsoft, dell, and hp).