> I don't know what you mean by a choice
> of XML schema languages. I think the choice has to be XML Schema from
> the W3C. The only other choice is DTDs, and they are not expressive enough
> to foster interoperability or allow for complex, reusable data types.
I agree DTDs is off the table. To me the choices are XML Schema from W3C and RELAX NG from OASIS (www.oasis-open.org/committees/relax-ng). There are pros and cons to each. OASIS is a group that is heavily involved in the XML standardization effort. Their focus tends to be more in the application space, while the W3C focuses more on the core XML technology space (the lines are blurry).
RELAX-NG is a *much* more readable schema language than XML Schema and it is just as powerful for our needs. The downside is that I think at this point there are fewer tools to work with it; but I also believe that more tools are working with RELAX-NG. See the web page listed above for a set of tools.