I believe Tablets are way more than super PDA's or laptops with pen input. Their vertical format is a big plus. This opens up a possibility that tablets could be a tool that could replace textbooks and enable textbooks to include time based media. IMHO the big problem with e-Books, as they were first implemented, was implementation of them on computers that included such non-book things as horizontal formats. Having keyboards on on a laptop or notepad made e-Books much less convenient than books, even when trying to view a page as a vertical. This is not an issue with a tablet.
Also, tablets could replace paper notepads. Paper notepads are still the standard note taking tool during class lectures. For a lot of folks today paper notepads are a better way to take notes during lectures than laptop computers based upon keyboard input. Some people don’t keyboard as well as they write, plus keyboarding limits people to just text input. Tablets are better than laptops because of the ability to input data in dynamic ways. As well as inputting text you can draw, and circle text and objects. With a tablet computer you can create objects and draw lines between things that define relationship as you input. You can embed time based media, such as sound or video clips into your notes. Isn't that what multimedia is all about? A tablet computer is more portable than a laptop and it is like having a built in Wacom pad. I think notepads can do for input what graphical displays did for output.
I think that tablets could replace laptops. They include the ability to do what laptops do and so much more. With docking ability they can function as a tablet, a laptop and a desktop! It is hard to use a laptop while standing and walking. This limitation forces us to design labs with students anchored to chairs. This, in turn, limits collaboration.
I think tablet computers are real "disruptive technology" (as defined by Clayton Christensen in his book the Innovator's Dilemma) that just needs vision and evangelism to take off. I am really looking forward to getting my hands on one and showing folks what I think it can do!