WP8 apps can resume from the start screen or app list now... only catch is that the app has to be updated for this to work. It's an easy upgrade, but still - would be nice if this were also a feature in 7.8.
Windows Phone 8 introduces the ability for apps to request that user actions that would typically relaunch the app, such as tapping the apps Start Tile, instead resume the suspended instance of the suspended app instance, if one exists. This feature is called Fast Resume.
Wasn't this documented for WP7.5 as well but never did resume from the start?
Documented was fast app switching. "Fast resume" was always handled during compilation of the app, so developers had no control over that (they could go modify the XAP when it was done, but I doubt that would have been accepted into the Marketplace).