I experience this flashing on my MacBook Pro as well. I experience it in Safari, and in Chrome. I notice it much more often when I use my external monitor, but it also happens using the integrated display. I think as it changes pages, the screen first loads a white page, then fills the content an instant later, causing this flashing effect. It's nothing during the day, but at night it can be sort of obnoxious. It is clearly a software issue, hope it goes away in Mountain Lion.