Apple doesn't want your grubby hands mucking up their pretty computers.
There's a wonderful little website named iFixIt who does complete, guided teardowns of new products as soon as they launch. They cover every Apple product that launches, and I've noticed something after following them for a while. The iMac is very easy to teardown, with many steps and tasks completable by amateurs. But the story is very different for devices like the iPhone 4, which Apple has gone out of their way to make inaccessible.
The iPad is also very difficult to get inside- it has a 4/10 repairability score, compared to 7/10 for the iMac. The MacBook Pro got a good score as well- another 7/10. Which makes me think there's a fairly clear divide between Mac and iOS devices in terms of their design philosophy.
Which shouldn't surprise anyone. Both product lines are handled by different teams and are aimed at fulfilling different needs. Only that isn't as true now as it was two or three years ago. iOS and OS X are moving closer together. The most recent MacBook Air was built with the iPad very much in mind. This 'mobile Mac' has been an extraordinarily successful product for Apple. And it has more in common with the iPad than instant-on and an extremely convenient size.
The Air is also very hard to teardown. In fact, it has the same 4/10 score as the iPad. The iFixIt staff noted that "it's clear that Apple didn't want people to open their machine".
The Cupertino-based company has always liked exerting as much control over their products as possible, but that tendency has turned into a full-blown obsession. The iMac and MacBook Pro won't be spared for long. In the MacBook Air announcement, Jobs called the new Mac the future of notebooks. And, presumably, the future of MacBooks. Small, efficient, and impossible to tinker with.
After all, why do you need to get at your CPU? Or your battery? Or the LCD part of your display? You'll probably mess it up. Just let Apple take care of it.
Let Apple take care of it, and everything will be alright.