Please don't disable the back/forward buttons. If you don't
want to use them, feel free not to. Don't break the browser functionality for everyone else.
I second that -- please, don't break the experience for the user. This is a standard premise of modern web development -- and a reason why pop up windows and browser shaking is frowned upon. Leave the users functionality to their choosing when it comes to their application, don't break standard things like the back button.