@ Matt Bryant
As I posted when El Reg first reported on this latest round of Mac tablet rumors -- which you may not have seen -- for me, it all depends on whether it runs OS X or the iPhone OS.
As a professional illustrator/graphic designer, having a tablet that can -- even adequately -- run my copies of Photoshop, Freehand, etc. means that I have a sketchpad for taking along when I go to talk with clients, to do quick sketches, mockups, etc., then download them when I get back to the office, With (ideally, wireless) connectivity and a basic office app, I could even leave hardcopies of the roughs with a cost estimate, all sent to the client's network printer while onsite. Remember that, as a sketch pad, it doesn't HAVE to be as powerful as my office machine; it just has to be good enough for what I need it to do when I'm AWAY from the office and save files in formats that I can open when I get back there.
As a visually-impaired, trifocals-wearing 50-something, having a large paperback/small magazine sized page for working/reading on is MUCH better than a standard PDA-sized screen -- I CAN read my Palm's screen, and I like having color, but, even having just a gray-scale display, my old Newton 2100's larger screen was MUCH easier to read.
As a non-driver who travels on public transit and has a lot of time to read, again, the larger screen would make reading e-books, NYTimes.com, etc, MUCH easier than on a PDA-sizes screen and wouldn't require that I carry all those extra wodges of paper around with me. Add in color, which the Kindle doesn't have, and the Zinio-style e-magazine subscription model for magazines that I currently buy in dead-tree format becomes much more attractive. And while it is certainly POSSIBLE to do all of these on a laptop, consider again the "visually-impaired" part, mentioned above. Reading on a laptop on a moving bus/train involves, for me, bending my spine into a pretzel shape in order to get within reading-range of the screen while the laptop is, in fact, on my lap, or holding it up in front of my face with one hand while scrolling with the other. Neither is an ideal solution for relaxing reading.
Now, I will admit that my particular needs make up a rather small market niche but, if the groups into whose intersection I fall are taken separately -- people who would find a portable, networkable drawing pad/notebook ideal for work or school; people who want a large but still portable color-screened e-reader; and the visually-impaired who find reading a PDA/smartphone screen an exercise in frustration -- I would guess that you're beginning to get into a commercially-viable market demographic. And that's even BEFORE you get into the group of people who might want it just because of any perceived "coolness-factor"! ;-)
As noted, this all works for me if it runs OS X. If it runs iPhone OS, then probably not so much. It depends on whether the larger size and -- presumably -- larger storage, makes it an attractive enough platform for developers to port OS X apps to or to create adequate equivalents for (much like Seascape/Inkscape are to Photoshop/Illustrator). If it does, then it might suit my needs and I'm there. If not, then I'm probably not.
Does that explain it for you?
(Damn, that was a long one... Now where did I put my beer...?)