> As best I can tell, the functionality of Schematron + xPath 
> 2.0 is a superset of XML Schemas (and RelaxNG).

Yes, as far as testing validity is concerned, that's almost certainly true.
Any integrity constraint can be expressed as a query that returns true or
false. Remember however:

(a) a schema is more than a yes/no validity checking function

(b) some constraints are very convenient to express in the form of a
grammar, and these constraints are far less easy to express in terms of
XPath queries

(c) XML schema is carefully designed so that its constraints can be
evaluated in a single sequential pass through the document, without building
a tree.

Michael Kay
http://www.saxonica.com/