I've found several things to like and several not to like about Adobe's XMP specification (, , ), a subset of RDF that lets you embed standard and custom metadata into the kinds of file formats that Adobe products typically read and write. I recently learned from a Tony Hammond posting on the crosstech blog that Adobe just released a new version of this seven year-old spec. There's a new SDK with it as well, and while I only remember the SDK supporting C++ before, this "version" includes Java support.
I quote the word "version" because, as Tony pointed out, this new one has "no version number and no date". Hopefully they'll learn something from the current problems being discussed about dating releases of the XML Recommendations and the value of making it easy for people to cite specific releases of a spec.