Given some XML:
...the problem is you can't constrain the contents of
<elem>based on the value of the type attribute.
You can do it though, if you add an xsi:type attribute to it to explicitly set its type:
<elem type="typeA" xsi:type="elem_typeA">
<elem type="typeB" xsi:type="elem_typeB">
with suitable type definitions in the schema:
...and when the XML is validated the relevant definition will be used.
This technique is far from ideal as it involves modifying the source, but only in a way which doesn't break it for anyone else. Given the various options for validating co-constraints, this could well be the most straightforward way (at least until 1.1 comes along).