Calculating lat/lons from PostGIS table which has projected coordinate system?

I think ST_Transform is your friend.

Try this, it should work:

ALTER TABLE gisprod.buildings ADD COLUMN lat double precision;
ALTER TABLE gisprod.buildings ADD COLUMN lon double precision;

UPDATE gisprod.buildings SET lon = ST_X(ST_Transform(centroid(the_geom),4283));
UPDATE gisprod.buildings SET lat = ST_Y(ST_Transform(centroid(the_geom),4283));