Then there's the OS. The one huge failing in my mind is the lack of offline capability. No Internet = useless brick. That is supposedly coming this summer.
But what software/activities are you planning to use? I can give more detailed thoughts. It's great for light writing, blogging, web browsing etc. Easy to post photos via the SD card slot though less great for photo editing.
In your case, it seems to be an either-or trade-off. I'd recommend an iPad 2 ... great games and apps, decent web browsing experience, pretty good for note-taking. I was worried about the on-screen keyboard myself. However, the larger screen real-estate makes the keyboard very usable (at least for me).
In summary, its a very useful tool, but absolutely not a replacement for my MBP or T400.
Can someone please tell me if I'm mistaken?