When we use an embedded iframe, we can't test any applications that use "frame busting" code to break out of frames and make themselves the top-level application.

Selenium Core should use a pop-up instead of an embedded iframe; this should be the default mode so we're sure we get adequate test coverage. Note that it can be trickier to detect page loads in a pop-up than in an embedded iframe; that means that it's really important that we guarantee that Selenium handles these situations robustly.