Another timely Paul Thurrott reality check; read the full post
Put simply, when you use an iPad, you're typically not contributing to anything, as you can on a PC. Instead, you're simply consuming. And this is how I think the iPad should be compared to the PC: Consumption vs. contribution. Yes, you can do things like answer emails (using the virtual keyboard) on the iPad; there will always be exceptions to any vague generality. But for the most part, that's what this is about. Consumption vs. contribution.
When you go out and about with just an iPad, you're sending a message that you're not going to contribute. You're just there to consume. This is why the iPad is, to my mind, uniquely unsuitable in the workplace. Knowledge workers don't just read documents. They comment on them, edit them, send feedback. They contribute. And contributing means using a device that not just allows editing, but makes that capability a central point of the entire experience. (Multitasking wouldn't hurt either.) The iPad is not a business tool. In fact, for most people, it never will be. (And those who contort their workflow to make this possible are, of course, simply trying too hard to justify their vanity purchase.)