Windows Phone 8 Multitasking finally explained (Video Inside)
There has been a little confusion about how multitasking works in Windows Phone 8. One big complaint that many people have had including myself is that when you launch an app from the home screen is always relaunches the app instead of resuming the app. A build session finally explains how this whole thing works once and for all.
If you watch the video in the link the guys in the video starts explaining thing at about 9:20 minutes in with a demo at 11:00 minutes. In summary it is possible for apps to resume no matter where they are launched just like in iOS and Android BUT it is up to the developer to enable this behavior.