An input interface you have stare at to have any idea of what you're doing, constantly repositioning your hands while you do to see what's behind them... I am a happy iPhone user, so I feel like there are some good ideas supporting the use a touch screen, none of which are related to ergonomics.
I hope I'm not burying my head in the sand by saying that laptops are going to stay until there is something more handy that can viably replace their functionality. If all you do is spending short stretches of time using single-application drag and drop work, email reading or browsing, sure, an iPad might be your better choice. If you're using your computer actively for more than 8 hours a day, doing any typing, rely heavily on side-by-side windowing, or mind paying for even the most rarely useful application, you aren't going to trade your laptop for an iPad. Chances are you aren't even going to trade your desktop computer for a laptop.