I strongly urge you to go to heroic lengths to prevent breaking the back button, it is a sure fire way to alienate your users and even made it to No.1 on Jacob Neilsen's Top 10 Web Design Mistakes in 1999.
Perhaps you could consider rather asking the question: "How to avoid breaking the back button for ?"
If Scott's answer hits close to the mark, consider changing your flow to the PRG model. If it's something else, then give a bit more detail and see how we can help.
