The webpage at http://www.pixeltree.org/code/ identifies the new 'PCST' (or 'PCS') tag as what iTunes uses to identify podcasts. However, beware running the code that the author supplies to overwrite the tags--I don't believe that it is OK to just zero out unwanted tags.

Thanks to David Hull for the link to the code at pixeltree! It compiled simply on Mac OS X by saving the code and then typing "gcc fixid3tag.c" (w/ Developer Tools installed).

I even modified the code so that it could run on any number of files at once, instead of one at a time.

Works like a champ! :-) The resulting files/tags are easily displayed by iTunes and the "id3v2" program mentioned here, so I don't think there's any issue with the zeroing out these tags.

Nice job!