This seems to me like another instance of the all-in-one design fallacy (about which more in All-in-One crap, iMacs pt deux and Why the iMac design irritates me).
Sure, now you will tell me that being all-in-one is precisely what is so great about it.

But I think that, currently, computer technology has not reached the maturity (or stagnation) level of others. For example, in internal combustion cars, a thoroughly solidified technology, you never think about expanding the engine.
Nor are computers so tightly integrated (like a piano) that thinking in terms of modules and interchangable parts stops making sense.

My take is that I want to extract five years of use from a computer. This implies a mid-life expansion. How convenient is that ? Does it imply, as in this case, throwing away my current hard disk and migrating to a bigger onw ?
I also know that computers will be used in ways that cannot be anticipated, one of the reasons being that in two years we will have on our desktops technology that currently is being invented.
So, suppose that in 2002 I want to install Myrinet, because it has become both cheap and necessary. Wouldn't I be totally fucked ? Will there be (and at what price) some grotty hardware hack to allow a Myrinet NIC into that oh-so-beautiful and small transparent box ?

It is surprising the way Apple Computers keeps making the same fscking mistake, design-wise. Of course, it is a mistake in a PC kind of sense, that's to say, it is bad design for a computer that has to operate in an office environment on a typical user's desk.
Clearly it is good design for an expensive desk toy. It is probably a good design also for someone that is so technologically challenged that the idea of expansion is beyond them.