I've been working on some extremely complex web-applications and supporting the back-button in all scenarios is non-trivial, to say the least. This is especially true when you support various devices and browsers.
So, the question is, will it ever be acceptable to disable the back-button, within a complex web-application?
Data Point: We did some trials, where we got a few dozen users to work with a version of our application that does disable the back-button and they very quickly adapted (on average, 30-60 minutes, based on feedback).
Of course, you have to provide all appropriate navigation options, but it certainly makes life a lot easier, as there's a lot of edge cases you don't need to care about.
I welcome any feedback and suggestions. Thanks.