I would prefer if the back button only worked within one application only, and not across applications. That doesn't make much sense to me, and I do find it confusing to use.

That being said, I wish they just got rid of the buttons and implemented UI gestures so they don't have to waste screen space with the buttons and also to make it more intuitive. I like some of the ideas in the N9's UI, where a simple swipe of the screen can get you back to the homescreen, and so on:

http://www.youtube.com/watch?v=nl3VOAf15BU&feature=relmf...