Cached map services do not display when using the REST API, but appear in clients making an Internet (SOAP) or Local (DCOM) connection to ArcGIS Server.
The HTTP response code that correlates to the failed request is 404, as it can find the host, but not the file.
The cache tiles are stored on a server that is separate from the Web Server, and authentication issues do not allow the REST tile handler to access the file on disk.
The Identity used in the article is the ArcGISWebServices account, which also needs to have at least read permissions to the cache tiles as well. When using local accounts, the ArcGISWebServices user needs to be created with the same password on the server hosting the cache tiles.
If the ArcGISWebServices user does not have proper file permissions to the cache location, it may take several hours for Windows to apply permissions to the many files contained in the directory.