From the T web page:
Blue Line service is being diverted due to medical emergency at Revere Beach Station. Substitute bus service will replace regular train service between Wonderland and Orient Heights until further notice. Please allow extra time for your commute. 5/8/2009 10:41 AM
As of 11:50, the bussing is still in effect. No ETA as to when normal service will be resumed. Must be a serious incident at Revere Beach, as the delay actually made the WBZ traffic reports as well.