How about combining BB's glass shader with lesbentley's suggestion and then plugging either a "Spots" node with a Spot Size of 1.0 or less and Softness of 0.2 and Threshold of 0.6 or so with colors white and black into displacement or simply making a displacement map with dots (some distorted like rain drops) and plugging that into displacement... I'm sure it's possible... I think it would just be a matter of getting the displacement values right and also getting the Fresnel settings right also.