I noticed this yesterday on my Focus. I rarely use Zune at all but had my phone plugged into my laptop and was trying to get an App's live tile to work (it stopped working after I forced the 7.8 update). Anyway, every time I would try to power off my phone, the Samsung screen would appear and it would boot back up. I thought maybe the 7.8 update broke something and my phone would no longer shut off. I unplugged the USB from my phone and powered off and it shut off normally. So having the phone unplugged from my laptop seemed to work.
Not sure if this is how it has always been, since I rarely use the USB to charge it or if it's something new to 7.8.
Sorry if this is not the same issue that you are experiencing!