It's hard to imagine why they'd want a cloud-based OS. It has, as near as I can see, only downsides and no upsides for them.
It would piss off customers, create a poorer experience, make their laptops worthless in places where no wi-fi was available, cause anyone who is concerned about security not to use Apple products, and force Apple to maintain enormous data centers all over the world just dedicated to nothing but pumping out massive quantities of bandwidth for every Apple computer. And for what? They want to sell boxes, not operating systems.
SSD drives are becoming tiny. Have you seen the SSD drives in MacBook Airs? They're barely the size of a RAM stick! Moving to a cloud-based OS wouldn't make their devices any smaller; in fact, portable devices would likely get larger, because they'd need to use more bandwidth, and that's power-expensive for laptops, tablets, and phones.
Even the Adobe Creative Cloud apps live on your hard drive and keep their data on your hard drive. They don't actually run from the cloud, per se; the information that passes over the network is (mostly) license-checking information.
