I wrote back in March about how to decide what width your website design should be. The decision usually comes down to what we know about the audience in question, and what size monitor and resolution they are likely to use. In the best cases, we have actual data for the specific audience in question to go on, but that data can be sometimes be hard to come by.
Adding a wrinkle to the question is that, as more and more people start using larger monitors, they are not all necessarily going to maximize their browser to fill the entire screen. Some people will be resizing their browsers to either make more room for other windows, or to exercise more control over the width of pages that they read.
While I continue to hunt for a more reliable reporting of these numbers, I found this article from Roger Johansson following up an informal survey of visitors to his blog 456Breast.com in which he found some interesting data about how his readers browse the web.