I've had a Tosh M200 Tablet for the last 5 years and it has now travelled the world with me. Originally bought as a high-end laptop/tablet with hi-res screen (12" 1400*1050 - can't get that any more!) it's spent more time with me than my other half, much to her annoyance...
In my experience there are a few things they will have to ace if this is to work:
1) Hi-res display: The major failings of most current laptops/tablets are the low dpi of their screens. It's the reason I can't replace mine, the current crop of widescreen TFTs are a poor 1200*800 max, which frankly has pixels the size of my teenage spots. The whole point of a tablet is that when you write with the stylus, you can't SEE the individual pixels - it's supposed to emulate paper, not an etch-a-sketch!
2) Usable outdoors, in daylight: Mine is great indoors, but useless in sunlight as the backlight ain't nearly bright enough too, so needs to be transflective or something.
3) Bloody good Voice Recognition: Handwriting recognition in XP amazed me with it's accuracy when I got mine, even though I can barely read my scribbles, so that's good to go already. Voice recognition is way harder, and a pain to train as well. Needs to be highly context sensitive to work well e.g. read, red, Redd, etc. The M200 has 3 mics on the screen to do noise cancelling, so an apple tablet would need a similar setup to work well. Unfortunately MS in their stupidity broke the VR in Office 2007 and said F&$k off and buy Vista if you want to use it again...
4) Weight: Mine is <2kg but still starts to strain the arm when writing for awhile. Theirs will need to be ~1kg for comfort. Optional Bluetooth keyboard option to save weight anyone?
5) Battery life: As a pen & paper replacement, it'll need to last long enough to get a day's work out of it - either very smart power management or amazing batteries are required.
/My two cents