AppSlide, a new Cydia tweak developed by chpwn, turns your iPhone or iPad’s home button into a back button.  The developer’s description explains it in more detail:

This tweak will use a sliding panel-like configuration when one app launches another app. Then pressing the home button will act to send you back to the first app without returning to the homesreen.

No new icons are added to your homescreen. Configure the Activator method for the appslide back button from your Settings app. (it is the Home button by default)

You can see the tweak demonstrated in this YouTube video posted by the makers.  ‘AppSlide’ is currently available in the BigBoss Cydia Repository.