I'm a big Paradise Lost fan and studied some Christian mythology. Michael is the head Archangel, leader of God's armies, and his name was the battle cry against the fallen angels(Michael means "Who is like God?"). Besides being a psychopomp for human souls, his main job is being the nemesis of Lucifer and the fallen angels. If you notice in most biblical stories, God isn't someone who just materializes himself and handles things, he works through his angels and his people. Michael is who God works through to defeat Satan. Google St. Michael's name for a photo search, and I guarantee nearly every single one will be of St. Michael standing on top of Lucifer, shoving his face into the ground with his foot, and holding a sword or spear in the air in triumph. And this it was one of those pictures Zack pointed to when he said Dean was going to destroy Lucifer.
I could be wrong, but I think they've pretty much laid out whats going to happen. Sam will become the vessel for Lucifer, and Dean will become the vessel for St. Michael, and there are going to be a lot of tears for Supernatural's series finale.