For executives who spend most of their work-day running from meeting to meeting, the boons of constant connectivity, super-portability, and a large screen for reviewing metrics are a huge win.
I work at a major tech company. While the vast majority of the employees have no business using an iPad for anything other than iPad development, it's a staple among execs. It slips in a brief-case, can display large pretty charts/email/calendars, provides a better interface than a blackberry or iphone for answering emails, is pretty much instant-on, has 3G so they can use it on the road, and it can be passed around at a formal or informal meeting in a way that a laptop really can't.
I don't own one, but I understand why these people do. I also understand why other non-execs at my work have them; it pays to resemble the boss.
The other killer app that I don't think is fully realized is medical services. Nurses/doctors spend large chunks of the day going from patient to patient, reviewing files, and looking up symptoms. The last four doctor's offices I've visited have all had computers in the room for the practitioner to look stuff up on, and every single one of them does so. None of what they do couldn't be handled just as easily on an iPad, and the aforementioned portability/constant connectivity would be super-useful.