A few days ago I complained about not being able to categorize photos in Vista Media Center using metadata. I was using the 5384 (beta 2) build!
But in the most recent builds there are a new pivot called Tags, in which you can watch the photos organized by keywords instead of only by date and folders.
The tags can be edited through the Windows Photo Gallery and is persisted in the image.
Read more at Matt Goyer’s Media Center Blog.