At this stage in Apple's development as a company, a new hardware product needs to become a huge (billion-dollar plus) business in order to justify the investment, and more importantly, the opportunity cost, especially that of senior executives' time. When I've heard about rumors of an Apple tablet computer, I"ve always been somewhat doubtful that the potential market was big enough to warrant Apple's attention. This post is an attempt to inventory potential use cases and see if my opinion changes.
Gaming: bigger screen equals more fun. I can't really imagine playing SimCity or Civilization on the iphone. Both those games worked great, though, on my Mac IIci's screen. Any game that's played on the ipod touch or on facebook will benefit from being on the larger screen of the Apple tablet.
Music: ipods are great for solo use, but not for a group. Also, live DJ'ing with an ipod sucks, since finding songs is difficult and there's no way to manage a song queue. If the Apple tablet supports itunes DJ I will buy one just for that reason. Also, an apple tablet seems like an ideal place to enjoy itunes LP covert art and bonus features. Who knows, maybe the tablet will even be a good place to clean herb (as the jacket for Rastaman Vibration famously proclaimed.)
Magazines/newspapers: one of the things I was surprised about with my ipod was how much I read on it, especially the NY Times app. With the app store, Apple finally has a payments system that will work for content on the web, since they've made it feasible to charge a small amount (1-3 dollars) on a one-time basis. Nobody but nobody wants to sign up to be charged on a recurring basis for internet content. They will, however, buy it on a one-time basis when they are especially interested or know they have time to read it (like stocking up on magazines at the airport bookstore before heading to Hawaii.) Eventually, if they like something enough, they will have the opportunity to get a subscription, at which point the recurring charges will feel like a feature (convenience) instead of a bug.
Books: Steve Jobs has pooh-pooh'ed the idea of Apple making an e-book reader almost as many times as he said that video wouldn't work on an ipod. Amazon's kindle has validated the e-book space, and there's no reason why Apple can't make something 90% as good that will also do much more.
Web surfing: with a larger screen and more usable keyboard, the tablet becomes just as good a web-surfing experience as a Macbook.
Kids: most tech analysts are angry, child-hating loners, so they underestimate the kinds of factors that parents look at when buying a computer. If the tablet is solid state with few ports and a touch screen, that means it will be very hard to damage, and will survive dropping much better than a laptop since it doesn't have a hard drive. Also, the tablet will be much more locked-down than a Macbook, and thus parents will feel safer giving it to their children or teenagers.
There's probably a lot more use cases, but that's enough for now. Having written these, I am now agreeing with John Gruber's analysis. The Apple tablet will be positioned as a universal personal content consumption device, and for many users in the consumer space will replace the Macbook.
Predicted price breakdown:
64gb SSD: $899
128gb SSD: $999
(it won't be cheap when it first comes out--no important Apple product ever is.)

A lot of people are saying what you're saying about there being "no reason" it can't do what the Kindle does 90% as well. Us loyal kindle readers may disagree, though - one huge attraction of the kindle is the special non-backlit screen. I can't read on the iphone or a computer for prolonged amount of time (my eyeballs start to hurt). This to me is the number one reason why the kindle pwns. So unless the tablet is going to have a special screen that can both be a normal screen AND a good e-reader screen (which was rumored at some point), I'm skeptical on the iTablet as eReader part...to me it would be more like 10% as good, rather than 90%!
Posted by: Andrewbhall | January 04, 2010 at 03:00 PM
@andrewbhall: valid point.. i've not ever used a kindle for extended periods, so my ignorance shows in what I wrote above. I wonder how hard it would be for Apple to have the tablet go into a special non-backlit reading mode?
Posted by: Xris Ernest Hall | January 04, 2010 at 03:10 PM
I'm actually beginning to think they'll have a higher end model - $1200-$1400.
Posted by: David Jacobs | January 05, 2010 at 10:14 AM
@Djacobs: that's possible, if they also offer a model with a 256gb SSD. But I'm guessing they won't, because when Apple announces a product they usually only offer two models, at most. When the ipod first came out there was only the 5gb model. There were only two iphones when that was announced, etc. Again, I'm just extrapolating from what I know of Steve Jobs's history. He likes things simple. So, my prediction is one screen size, two price points based on different amounts of memory, just like the iphone.
Posted by: Xris Ernest Hall | January 05, 2010 at 11:19 AM