I have been working a bit to get horao working to visualize PostGIS 3D data.  In the mean time, I have a little work around for viewing 3D extrudes a 3Dish way (i.e. isometric)

The nightly build of QGIS now allows you to visualize polyhedral surfaces, the output type of ST_Extrude.  So, I cheated.  ST_Extrude takes 4 parameters:

geometry ST_Extrude(geometry geom, float x, float y, float z);

an input geometry, and extrusion along the x, y, and z axes.  So, instead of a normal Z extrude where the x and y axes of the extrude are zeroed out, e.g.:

CREATE TABLE buildings_extruded AS
SELECT gid, ST_Extrude(the_geom, 0,0, 40) as the_geom
	FROM building_footprints;

We’ll extrude a bit along the x and y axes:

CREATE TABLE buildings_extruded AS
SELECT gid, ST_Extrude(the_geom, 20,20, 40) as the_geom
	FROM building_footprints;

And then view in QGIS with the original geometry on top: