Though not an optimal solution, since the ZIP code is not an included field in the results, it would be possible to find the ZIP code by running an additional query for each of your results. In the following query, change the text to be the desired latitude and longitude.
SELECT postal FROM geo.placefinder WHERE text="37.389001, -122.003952" AND gflags="R"
To add insult to injury, YQL does not currently support table joins...at least not in the typical sense, so it's not possible to incorporate the above query into your current query.
For reference:
geo.placefinder parameters: http://developer.yahoo.com/boss/geo/docs/control-parameters.html