Well, as I blogged on our web publishing blog Nascent we just went live with XMP labelling on Nature in yesterday's double issue. We will be adding XMP to all new issues of Nature as well as rolling out across all our other titles in the next few weeks and months.

The screenshots below from Acrobat (File > Properties, CMD-D / CTL-D) show what the user might see both with (bottom-left) and without (top-right) semantic markup.

As to the actual contents of the metadata record, see this sample I posted to the semantic web list.