Clay Johnson (who is quickly becoming my new favorite blogger):

Remember: the key to having strong focus is the elimination of distraction. While a lot of space has its plusses, too much space is only creating room for more distractions. Having your mail, twitter, and IMs pop up in one monitor while you truly work in another is just giving your distractions equal ground to what you’re working on. Even having relatively static things up in extra screen space is a distraction. A todo list in a second monitor is nothing but a constant reminder of other things you could be working on other than the task at hand. Keeping anything up other than what you’re working on is a great way to keep yourself distracted from doing the important stuff you don’t want to do.