The official website of "La Vuelta" of Spain seems to have a partnership with Google, just like "Tour de France" and Google France in July :

- All the stages of the Vuelta on Google Earth : http://www.lavuelta.com/06/googleearth/lavuelta06.kmz

- the Vuelta LIVE with GPS device and Google Maps :
http://www.lavuelta.com/06/ingles/online/gps.asp?e=1

- the Vuelta on Google Video :
http://video.google.com/videosearch?q=owner%3AVueltaCiclistaES&so=1

Sources :
- http://www.zorgloob.com/2006/08/la-vuelta-2006-sur-google-earth.asp
- http://www.ojobuscador.com/2006/08/25/vuelta-ciclista-a-espana-2006/

Weird, when I overlay this on Google Maps...
http://maps.google.com/maps?q=http://www.lavuelta.com/06/googleearth/lavuelta06.kmz

... I get "could not be displayed because it is not a valid KML or KMZ file."

yes because there is many "images-layers" in this KML file, such as the Vuelta logo.
And all theses stages are filed in directory, there might be a issue with that.