I have a 3 monitor setup at home. Both the WPF & Web (ASP.NET/Html) designers have a "split view" capability which allows you to see markup and the design surface at the same time (and keep them in sync). If you "split" vertically and stretch VS across two monitors, you can have your markup on one monitor and your design surface on the other. I just started using this feature and MAN OH MAN is it a UI developer's dream! Here's a screenshot of the web designer split across two of my monitors (sorry about the camera phone pic quality):
For WPF, switching vertically is obvious. However, for the new web designer it is a little bit more hidden. Scott Guthrie has an explanation of how to enable it here.