I think the future is clear, and it ain’t spelled “XSD”. Major recent RELAX NG wins include DocBook, OpenOffice, XHTML, and SVG; all of which are planning to move to RELAX NG in their next versions. I have yet to encounter a group that seriously explored RELAX NG and still chose to use the W3C XML Schema Language
I have only taken a casual look at RELAX NG, but I clearly like it a lot better than XSD. Now if it would only be acknowledged in the Web services world …
Looking at the examples in the RELAX NG Tutorial, http://www.relaxng.org/tutorial-20011203.html, it seems to have one really big disadvantage: it’s impossible to read.
I can take in the DTDs with a glance, but I really have to work hard to understand the equivalent RELAX NG.