Kickaha used the word that I've been using with people when I talk to them about the future of UI and computers: appliance. That's what the iPad really is, except that it is plural. It does one thing at a time, and it does that one thing really really well. When you want it to be something else, jump back the the home screen and pick what you want it do next. The key with appliances is that we don't really care, or need to care how it does what it does on the back end, we just need the easy to use controls on the front end. The possibilities are immense.
When you want an 'appliance' to write a letter, open Pages. When you want an 'appliance' to listen to music, open iTunes. And if Apple can rewrite these apps to function this well, what's stopping other vendors from doing the same thing?
Get some TV and stereo components that can take command via Wi-Fi and this thing could be the most amazing universal remote control ever.
Hell, extend that to other appliances in the house. Imagine an oven with a built in camera and Wi-Fi controls. You chould check the pot-roast from the living room and adjust the temperature.
You could monitor security cameras or baby monitors on a decent sized display. You could control the lights remotely, set the DVR, and on and on.
I don't agree with anyone who says this thing isn't a computer. In fact, I think the iPad helps makes computers to be even more 'all that they can be'.