There have been some issues with the exchanges lately. Mt Gox's market share has dropped substantially, and its price has diverged from the other exchanges. Many buyers buy from exchangers who base off of the mtgox rate, while the sellers prefer the other rate. So, we have to compromise. Until further notice, we will use the current average of mtgox and bitstamp, the two largest exchanges and update every hour. I hope this is agreeable to everyone.