I think the MacBook Pro 15" is pretty light already, I'm already content with it. If I were to get an iPad, it would be like a downgrade, at least that's how I would feel about it.
I would never be able to replace my laptop with an iPad, simply because I'm a software developer. But I'd certainly buy it as a portable mail, web browsing, movie and ebook device, which just happens to fill all of my parents computing needs.
So while it's not going to be my main device, I certainly see it as being a competitor to the traditional laptop for a MOST people.
Most of all, I think Apple has managed to get the "magic" right once again. I never told my parents that they should buy iPhones, but they did after trying mine, and they are very happy (I'd never thought that my mother would start browsing the web and sending emails, but it's happened thanks to the iPhone).
However a MacBook Pro with an iPad is a very sweet and flexible combination.
Look at what happened to the iPhone in 2 or 3 releases. For heaven's sake, Apple was able to establish a game changing ecosystem my then. Extrapolate that for the iPad. There's a good chance that the 3rd generation iPad will be able to subsume 90% of laptop use for almost 90% of the population.
If you want to evaluate the challenge and potential windfall facing Apple with the iPad, then don't compare it to the iPhone. Compare that to the establishment of the App Store.
