I carry my 13" MacBook Pro every day. I make home visits to clients and have a suite of presentations, elements of which I may need to show them. I have a couple of spreadsheets into which I need to enter data about twice per week at a remote clinic. My iPhone serves to take brief notes and remind me what's next on my calendar, but I prefer the month-at-a-glance view in iCal on my Mac for booking new appointments. I have a load of textbooks in PDF form which I can either read sitting at my desk or through a window on my MacBook (unless I turn it sideways, which is ungainly).
The iPad is one-third of the mass of the MacBook Pro: it would allow me to show my presentations at a size my clients can see, run my spreadsheets, has a usable full-month calendar display and can show PDF books & magazines at roughly "actual size" when I'm sitting on the sofa or on a train without the awkwardness of a keyboard hanging off the side of the document. I have a sleeve case for my MacBook Pro: the iPad would fit nicely into a similar -- possibly even the same -- case while I'm on the move.
There are still many tasks for which I would use a full Mac, but most of those would be desk-based activities (e.g. programming, extended writing). I could see myself replacing my MacBook Pro & Cinema Display combination with an iMac and iPad, or even just leaving the MacBook Pro docked to the Cinema Display most of the week (but with the option to go mobile if I need extra power with me, e.g. when I'm travelling abroad).
The MacBook Air wasn't for me, but the iPad meets *my* needs exactly.