There's a long way to go before this is over and he's not one of the top ministers, but this is still a big development. BBC News:
Abdo Hussameddin announced his defection in a video posted on YouTube. "I am joining the revolution of the people who reject injustice and the brutal campaign of the regime," he said. He is the highest ranking civilian to abandon the government of President Bashar al-Assad since the uprising erupted a year ago.