I look at the back button as being synonomous with undo as well. So I can understand it being problematic to trash a web app when you hit it.

At the same time, there are some pretty clear ramifications of allowing page control over more portions of your browser. Anyone remember the right click protect code? You right click a page and get a dialog box that says "LOL DON'T STEAL MY IMAGES" or whatever. The next thing you're looking at is someone injecting ad pages into your history, or not allowing you to go back at all.