The ipad is not a computer, it's a digital media appliance, as is most every modern phone, gps, e-reader, digital media player, and dedicated gaming platform on the market. This class of function-specific digital devices is about reliability, user experience and performance. A controlled operating environment attempts to consistently deliver those things by restricting the entropy induced by random user code. Boiled down, unbounded tweak-ability = break-ability. Do most consumers want to debug an appliance every time they add a feature?
If you want a general purpose computer, use a general purpose computer. I want my game box and my phone to just work, as the iPhone does. I want some simple customization THAT DOESN'T BREAK ANYTHING, which I get now from the app store. It's exactly the right model for 95% of the user base who just wants the d@mn thing to work. GPS units proved it. The ipod proved it. The iPhone proved it again.
If I can get a fast web browser and digital media player on a cheap, reliable, gorgeous large-screen internet appliance that works through an entire trans-continental flight, without the liabilities of a conventional 'open' operating system, why the h3ll not? Sign me up.