After seeing how my blog posts have been republished without my permission, I realize that the same thing is likely to happen with my photos (and most likely already has happened). Most photographers add a visible watermark or copyright notice to identify their photos. Although Aperture can embed a copyright notice and author info in the EXIF data, it doesn’t have a way to add a visible identification.
One way to do it is to edit in Photoshop and add a text layer. As an alternative, I started to write an Aperture editing plugin that can add text to a photo. I hope to have it ready for public release next week.