Let's think about the many other attempts from non-apple manufacturers to sell tablets and how they failed?
A tablet is so cumbersome to handle. How do you hold it, using your right hand? left? Place it in your lap? On the table? But then your view does not fall perpendicular to the screen. How would you type? Using both hands?
I am sure apple is smarter than that, and all their products have been innovative and kick ass, they wouldn't do just a tablet... maybe a two screen tablet/netbook using a touch screen interface that can be also used for a touch screen keyboard? I don't know?! But just a simple tablet would suck even if it comes from apple!
Anyways, I had to throw in my 2 cents...
OK, this has been debated to death.. The problem with tablets up to now is that they haven't been done right. They have been based on Windoze, lacking right ergonomics, lacking real multi-touch interface, unsightly, and just plain pain to use. If there is one company that can do tablet right, it's Apple. Remember Newton in early 90s? It had a very loyal fan base, but the device was ahead of its time.
So I think there is a huge opportunity here for Apple, and they have the chops to execute this well.