When is opening a new window a good idea?
Because users have a 1600x1200 display, a convenient window size to see the content is 800x600. Multiple windows are a more efficient usage of this screen than a single bigger one.
It's very context dependent as yet, but we are getting to situation where multiple windows are useful. IMHO, we're going to see valid uses for multi-window content becoming more and more popular for the desktop web.