As well as non-.NET-specific things like XSDs and XPATH queries:
You'd want to be familiar with the different ways of handling XML that the .NET framework has provided - the old
System.Xml namespace (
XmlDocument et al) from .NET 1.1, and the new
System.Xml.Linq namespace (
XDocument et al) from .NET 3.5.
You'd want to be familiar with serializing and deserializing objects to and from XML, the various attributes in the
System.Xml.Serialization namespace that affect that, like
You should know how to fight your way through dealing with XML documents with namespaces, since that crap never works cleanly in .NET.
You might need to be familiar with the
<asp:XMLDataSource> control, but let's hope not.
Having said all that, it's perfectly possible that "I know that XML has pointy brackets in it, and I can google the rest as I go" might be more than enough knowledge for many .NET jobs that list "XML" as a requirement. It probably would have been for several I've done.