SOLUTION: Add the "Version" parameter to your request.
EXAMPLE: http://webservices.amazon.com/onca/xml?AWSAccessKeyId=...&Version=2009-10-01
For valid "Version" values, keep your eyes on the API's release notes.
NOTE: By default, the API will return the top 5 tags. To have more returned or to page through tags, check out the TagPage and TagsPerPage parameters in the ItemLookup documentation.
