Aerodynamically speaking, a tail stall occurs when the eddy produced by the ice on the tail moves far enough aft to interact with the elevator thus producing a control buffet. For this to occur, you generally need a higher speed than would be required for a wing stall, even above the elevated stall speed for the wing in such conditions. This video may be a bit dry, but it has some great info on tailplane icing.
http://video.google.com/videoplay?do...3060735779946#