Why? Imagine a 12" Iphone screen where you wouldn't even need a keyboard. All of your input would be multi-touch. This sounds great to me. I don't even have an Iphone or a Touch yet and I think this could "revolutionize" the way we interact with computers.
Ever since I first played with one (an Iphone), I thought that this would make an incredible form-factor for an ultra-portable. The more I think about it, the more I am convinced that this could be another "ground-breaking" product for Apple.
Again, I too, think that this would not be a replacement for a main system but be a way to portable-ize everything you do on your home or office system. Granted, this may not be what they are actually releasing so I could be getting all excited for nothing!