The Metadata Working Group is an collaboration between companies in the imaging business, working to standardize the types and use of metadata. Founding members are Microsoft, Adobe, Apple, Canon, Nokia and Sony. The first report was released in 2008, and it has now been updated.
One of the most interesting sections of the new guidelines deals with both hierarchical keywords and other groupings (these are called Collections in Lightroom, and Catalog Sets in Expression Media). There is now an agreed-upon syntax for embedding those tags into image files. As software gets updated, we should see more use of these tags, and therefore more interchange of metadata.
This should help people both preserve and make use of their images and image metadata.
Anyone who is interested can download the new report here.