I downloaded all my gps traces.

Used a simple ruby script to convert them to a .osm file. Each gps point became a node with an amenity=post_box tag.

Used osm2pgsql to import the .osm file into PostGIS.

Cut down the mapnik stylesheet so it only rendered postboxes using a 1x1 black box.

Added a couple of lines to generate_tiles.py to delete blank tiles. Then generated the tiles for zoom 8 to 17 in the area of my gps traces.

The result can be seen here.

Perhaps I've just been unlucky but