your IP address may have been banned for excessive fetching of the said schema file - since DTD and schemas generate so much traffic for us, we've been pushing strongly XML developers to use a proper schema caching infrastructure w3.org/blog/systeam/2008/02/…

The blog entry is old but still applies; I can successfully wget or curl the http version, so it doesn't seem to be generalized problem (although it may still be more general than just you). Feel free to reach out to dom@w3.org if you want a more private discussion 🙂