Apple will be the market leader in slate form-factor computers in, oh, another day or two. There are already more applications written specifically for the iPad than have been written specifically for the Tablet PC in 8 years -- by at least one order of magnitude.
Although I'm dubious about the first-generation iPad (the screen resolution is my big sticking point -- 768 horizontal pixels is not good enough), there's no doubt that it will be an important consumer device. The question (well, one question...) is how important it will be in the enterprise. The easy answer is that it will be a good fit for certain vertical applications: I think you can expect to see a lot of iPads anyplace the insurance industry is involved -- hospitals, claims adjustment, etc. The more that walking around with a clipboard of forms is part of the job, the more appealing the lightweight slate computer.
But what about in the normal office? You still have vertical internal applications (I hope to be talking at the iPhoneDevCon about enterprise development for the iPhone and iPad) but another thought I had this morning in conversation with Alan Zeichick is that, once the hype fades ever so slightly, the iPad might be a nice device for always-on wall-mounted display for dashboards and calendars: