I went on a business trip this past week and decided to leave my computer at home. 98% of my work passes through my email, so it seemed that I could just take my iPad and get almost everything done. It seemed to work.
Two adjustments to the iPad were necessary. First, I purchased Apple's Pages program, an app that costs $9.99 and provides most of the full functionality of a word processor. Then I purchased the Apple Bluetooth Keyboard, a bit more expensive at $69, but weighing a mere 11 oz. Plus, it's gorgeous:
There were a few minor things that I couldn't do, most notably that I could not log directly into my company's network. But I could get almost everything I needed via email and Google Docs, so very little was lost.
All in all, a very satisfying experience. Laptops are doomed.