I think it's interesting so many of you mention the iPad as an eReading device. Don't get me wrong, I love my iPad (wouldn't trade it for an MBA, but I've got both anyway

), but its weight and (more importantly) display are major turn-offs for me when it comes to reading a book. I additionally got a SONY Reader PRS-650 which features an E-Ink display and is VERY lightweight. Reading on it is just wonderful, an experience the iPad (in my opinion) lacks.
I prefer the iPad for PDFs. I used to carry a ton of books to our pen&paper RPG evenings. Now I've got all rulebooks on the iPad, hence my RPG bag shrunk considerably (still have to carry dice and... uhm... well, pen and paper

), but the days of heavy lifting on the mage's side are over.
Furthermore, I agree on both devices being in their own categories. I take my iPad everywhere, even to my balcony for having a smoke (and it's really cold these days). Regardless of my 11" MBA's size, nothing beats the iPad's form factor in my book. It is however mostly a tool for consumption. Any time I have to produce something on the go, enter MBA (or MBP, depending on the tasks).
Every gadget has its purpose. To me, the iPad is my everyday all-the-time being online companion and I wouldn't want to miss it.
Sent from my iPad.