In looking at Adobe's XMP site, it references the Metadata Working Group (MWG), which has a definition for how to store face tags in XMP. (See page 51 of their PDF specifications). I was excited to see that someone has gone to the trouble of defining a spec, and it looks like they have done a great job of it.
The problem is, I haven't found any products that use that spec yet. Since XMP is Adobe's baby, Adobe could really help make this spec useful by supporting it in their own products, starting with Photoshop and Organizer.