The Problem with the iPhone's Home Button
I have a similar problem on my Droid X.  Sometimes the back button goes back a page (such as in a web browser) or while reading a message back will go to my contacts list (in google talk or google voice).But sometimes, in the same apps, the back button might take me back to the home screen (If i answered a message from the notification bar), or a previous app if something launched a browser.It's incredibly aggravating that the behavior changes, and especially so as Android devices have a home button!  Back should be constrained to a specific application.  Otherwise in google talk I have no reliable way to get back to my contacts and usually have to make a trip back to the home screen to directly launch the talk app.