i think if it was a slow, low-res (1024x768), keyboardless mac, then it would be stupid. there would be nothing new, and using it would be cramped-feeling and no fun. as others have pointed out, if you want a real OS on a small light device, you can just buy a small light laptop, and you get way more storage, computing power, pixels, etc..
and of course if you're geeky enough to want to do real computing on the ipad, then (a) you also have a real computer of some sort, and (b) you are capable of setting up a vnc connection to it. that way you can run your windowed OS and all the applications you already own on the same powerful machine that you normally use -- just using the ipad as a terminal. obviously there would be limitations and the experience wouldn't be that great overall, but i'd wager that vnc performance is still better this way than trying to run os x on the kind of hardware that fits inside an ipad.
the fact that the ipad is a fast, high-res (1024x768), keyboard-capable ipod touch makes it interesting and to me kind of exciting. i can't wait to play with one, because it does stuff that hasn't been possible or realistic to attempt on the iphone OS before now.
this looks like the perfect "grandma" computer (or "e-mail machine" as a buddy's grandma used to call it) and for someone like that it could be all the computer they need.
for the rest of us though it is probably a third device, iow not meant to replace either a computer or a smartphone. and any discussion of multitasking should keep in mind that usually you'll usually one of those other devices nearby.






