I believe if you use the date within the yweather:condition node that should give you the accurate "late updated" forecast. For instance, it's currently 8:10am PST and mine gave me the following result: <yweather:condition text="Partly Cloudy" code="30" temp="58" date="Wed, 03 Jun 2009 7:53 am PDT" />
So it was almost 20 min ago.
Hope that helps,
Jonathan LeBlanc Senior Software Engineer Yahoo! Developer Network Twitter: jcleblanc