All the examples on the page:
http://developer.yahoo.com/geo/geoplanet/result in:
<error yahoo:uri="http://www.yahooapis.com/v1/errors/400" xml:lang="en-US">
<description>400 Bad Request</description>
<detail>
Valid appid parameter required for this resource; please visit
http://developer.yahoo.com/wsregapp to get an appid
</detail>
</error>