Many people in the Bitcoin community heard about the shutdown of several US-based exchanges but few actually understood why.  Here is the specific document where the Financial Crimes Enforcement Network (FinCEN) laid out their definition of what a currency is, what a virtual currency is, and their understanding of virtual currency trading.  It was shortly after receiving this notice that banks that dealt with the trading exchanges notified them that they would no longer support their accounts, meaning that btc could no longer be converted back to dollars.  Like it or not, this ruling (and perhaps future rulings in the future) are going to have a profound effect on the virtual currency marketplace and community.