http://www.nasa.gov/...gine_icing.html - 17th Feb 12
For now, pilots are being trained to recognize the potential existence of these ice crystals, which are about the same size as baking soda, and advised to avoid the weather conditions as best they can. Although a potential hazard, no accident has been attributed to the phenomenon in the 23 years since it was identified.
It is expected that updated flight safety rules and engine testing standards will be adopted once all the research is compiled and analyzed during the next few years.
NB: this is ice inside the engine, not the airspeed indicators as proposed for the fault in the Air France F-447 incident.