Change or fix the back button concept.
There are many situations where I don't know where I land after hitting the back button, for example Internet Explorer and Zune.
Yes, when in IE, back button is back button. There's no way you can close IE directly.
Example: Open Zune app, go to a playlist and choose a track. Push home button, go open Internet Exlorer app. Hold back button a bit, to go to multitasking-view, tap on Zune-app where you see that it's playing your music. Than tap to the back-button, because you want to choose another track from you playlist... oh wait, where am I.
After heavy usage of multitasking it gets more complicated and confusing.