Thanks for catching that (ditto to Robert Leif, the earlier poster, who mentioned it). I've reported it to my XML.com editors and also to help@oreillynet.com; it should be taken care of within the next day or two. I'll check periodically myself as well -- if it's not here on XML.com by then, I'll post a link to the schema on my own Web site.
The link is still broken, and I'd really like to access the file.
The problem is a difficult one for me, and one of the most potentially helpful XML.com articles I've come across. However, I do not want to limit my solution to a DTD.
Please let us know where/how to get this sample, or how you'd change your answer if a schema had been requested in the first place.
Send your e-mail address to me (simpson "at" polaris.net) and I'll return you a copy of the Goldmine schema. (I'm not sure what's going on with the article link -- it is supposedly being fixed.)