"Did you really need a notebook in the first place? Or did you buy it just because at way back in the Pre-iPad Era, a notebook was the only mobile device available that could handle such a wide variety of tasks?"
Bingo! The very thing that I'm trying to figure out. I have a 12" PowerBook that I love, but have never used quite as extensively as I imagined I would. And while it certainly has come in handy while traveling, it's a bit more than I've ever really needed on the road. I don't need it for work, so it's useful mainly for entertaining myself on a plane or in a hotel room.
Of course, I'm setting this up as an either/or problem. Either an iPad OR a MacBook Pro. I did the same thing with the Kindle and the iPod touch. They're different machines that do different things. I still have and use the Kindle, and I liked the iPod so much I traded up to a full iPhone. It would be a chunk of change, but if I found I really needed both, I could probably find a way to swing it.
But, for now, while the ol' PowerBook still powers on, I'll probably go for the iPad at some point this summer.