For all those that see no need for disabling the "Back" button let me offer this scenario:

You have a web application that establishes a session and caches data based on what links you have clicked. If you then click the back button, the cached information on the server does not match the context you are now in. Thus, in this case, you would want to disable the "Back" button.

I have to confess, I have been temporarily "trapped" on a web page that disabled the back button and it has really annoyed me. Now I find myself developing a web application that requires this behavior.