Bill Gates comments on Tablets publishes a bit that got but out of its Bill Gates interview from PDC:

This is the Tablet, I don’t know if you’ve seen this Motion machine. [Gestures to a Motion Computing Tablet PC.] Just think of that two years from now, four years from now, in terms of the thinness, the cost. Every student, instead of having textbooks, they’re going to have a Tablet PC, and so the PC is very significant.

The problem is that slate Tablets have proved the least popular type, for a lot of users, and that’s nothing to do with the thinness or lightness. Anyone writing long documents – where long is over a few hundred words – wants a keyboard. For notetaking, reading, and annotating, though, a portrait tablet form is really comfortable.
So the obvious compromise is the convertable – and how slim and light a convertable can get is limited by the fact that it needs to include a keyboard and fold over itself.
One interesting point, though: everyone having Tablet PCs as students would make pirating text books much more likely…