web app - Does "Don't Break The Back Button" Apply To Web Applications?

I've read the advice, "don't break the back button" from multiple sources. For content-based browsing I agree with this 100%. Does the same advice apply to web applications? I feel that it is still valid for web-app pages that simply display data. Once posting forms, AJAX, and logged out sessions become involved, the back button might result in unexpected behavior.

Is this advice relevant for a highly-interactive web application?

Here are some articles that speak to this topic:
http://w3.org/QA/Tips/reback
http://brightorangethread.com/blog/view/dont-break-the-back-button

http://nngroup.com/articles/the-top-ten-web-design-mistakes-of-1999