Right now, the iPad is a secondary device for most people as it can't handle the heavy duty lifting and storage that is needed for media. I think that it is important to consider the way that laptops are just now starting to rival desktops in processing power because of advances in storage and speed that are not space dependent. How long will it be before a device like the iPad has the same processing power and storage capacity as a macbook? At that point, the decision is really going to be more about intended use. Are you looking for extreme portability or do you need to do a lot of typing and/or need a really big display?
Traditional PCs will likely always have a place in work environments where typing is the primary function...like at my office. They will also, continue to handle the heavy lifting and serve as a homebase for consumers...for those that need it.
I love my iPad and take it everywhere. I can't and won't do that with my personal Macbook Pro or my corporate work PC. I see a parallel to the line about the best Camera. The best computer is the one that is always with you.