It has been a long time since the "calculating distance on earth" debate. In fact, I was only able to find some notes.

Although, somebody asked for an equivalent in TSQL. I could not do this because I do not have access to this type of database server. Although, I am running PostgreSQL in a couple of locations and dug up this stored procedure (in plpgsql):

CREATE OR REPLACE FUNCTION zip_distance(integer, integer) returns float4 as '
 DECLARE
   zip1                alias for $1;
   zip2                alias for $2;
   v_zip1               integer;
   v_zip2               integer;
   longitude1          float8;
   latitude1           float8;
   longitude2          float8;
   latitude2           float8;
   delta_lon           float8;
   delta_lat           float8;
   temp                float8;
   dist                float4;
 BEGIN
   v_zip1 := zip1 ;
   while longitude1 IS NULL LOOP
      SELECT longitude_rad, latitude_rad INTO longitude1, latitude1 FROM zip_loc WHERE zip_code = v_zip1;
      v_zip1 := v_zip1 + 1;
   END LOOP;      

   v_zip2 := zip2 ;
   WHILE longitude2 IS NULL LOOP
      SELECT longitude_rad, latitude_rad INTO longitude2, latitude2 FROM zip_loc WHERE zip_code = v_zip2;
      v_zip2 := v_zip2 + 1;
   END LOOP;

   delta_lon := longitude2 - longitude1;
   delta_lat := latitude2 - latitude1;

   temp := pow(      sin(delta_lat/2.0::float8), 2.0::float8) 
            + cos(latitude1) 
            * cos(latitude2) 
            * pow(   sin(delta_lon/2.0::float8), 2.0::float8);

   dist := (3956 
         * 2 
         * atan2( sqrt(temp),sqrt(1 - temp)) )::float4;

   return dist::float4 ;

 END;
' language 'plpgsql';

zip_loc was a lookup table of zipcode longitude and latitudes. Actually, the final code used databases from US Census along with a modified version of the above (rewritten in Python), to find distances between street addresses. Also this data may have been cross referenced with USPS data. I no longer work on this project although I believe it is still in use.

If I remember correctly, this equation got me in some trouble with my meteorologist friends, Michael Tobis. Still, it's crazy fast and does the trick.