What is the point? WP7 is a 3rd party app single tasking OS, unlikeWindows 7 and 8 and Windows Mobile.
And some people claim they want it to tidy history screen. But, the history screen is only like 5 tiles long. And if I actually get to the point to tidy up my history list, I might as well just press Home and open the app from familiar places.
I rarely use back button for app switching. I use back button within the app. If I really need to swap between apps, I would hold the button. But, that is not frequent enough for me to micro manage my history list.