I haven't yet used either of these but PostGres earthdistance http://www.postgresql.org/docs/8.3/static/earthdistance.html http://www.postgresql.org/docs/8.3/st

I haven't yet used either of these but PostGres earthdistance http://www.postgresql.org/docs/8.3/static/earthdistance.html http://www.postgresql.org/docs/8.3/static/earthdistance.html or PostGis http://www.postgis.org/ http://www.postgis.org/ might be good options (if you don't mind leaving MySQL behind that is).