Case conventions on element names?

Most XML standards originating from the W3C tend to use lower case with hyphens.

There is a philosophical distinction between seeing XML as a format for platform neutral documents, which W3C standards try to encourage, and languages such as XAML which see XML as a serialisation of a platform specific object graph.

If you're not using XML as a platform neutral document format, but as an application specific serialisation, then you might as well save yourself some bother and have a 1:1 correspondence between the XML names and the platform specific names. But almost any other object graph format is better than XML for that purpose.

If you are, then you might want to fit in with XHTML, XSLT, SVG, XProc, RelaxNG and the rest.

Not that it matters, but I've always been partial to PascalCase for Elements and camelCase for attributes:

<Root>
  <ParentElement attributeId="1">
    <ChildElement attributeName="foo" />
  </ParentElement>
</Root>