Forecasting is a new capability added to Power View in HTML5. Just for fun, I pulled in the historical MSFT stock price starting from the year ( for better projection ) and tried out the forecasting and hindcasting. The interaction is indeed very fun and the capability is super powerful.
Here is the workbook embedded live. Try it out and Happy forecasting!!!. For more details, check out http://www.microsoft.com/en-us/powerBI/bi-blog.aspx#fbid=2yZXmU3hqQN