Xml.com is going gaga over XML Schemas. Schemas are meant to replace dtds as the mechanism for defining XML document structure requirements. To be sure, they are a huge improvement over dtds… schemas are well-formed XML documents (so you can use your XML tools to manipulate them), and schemas are infinitely more readable than dtds. In Microsoft’s XML Parser 3.0, they built in an object for caching schemas, which should help performance a good bit.
Unfortunately, there’s at least one thing that dtds can do but schemas can’t: reference external XML documents/fragments for importing. This is a big gap in XML, as far as I’m concerned.