Photographers need to be aware that there are some real concerns regarding how Apple Aperture 3, is writing metadata; especially when these files are viewed by other applications in an imaging workflow.
Those using Aperture to write metadata to files that will be used with other programs at some point in their workflow need to do some testing. Those sharing images with others directly, or via the Internet may find that even basic fields like the Caption/Description and Keyword fields are not visible after exporting from Aperture 3.
You need to understand what is happening if you do additional work in Photoshop or Lightroom, and then archive your images using other programs. Otherwise you risk having some or all of your metadata disappear. This is because other programs may not see (recognize) the information you are adding to your images with Aperture 3. In addition, if you are importing images that have been worked on in other programs, some of that metadata may not be recognized or stored within Aperture.
You should really be concerned if you are fully using the various metadata fields, such as the Rights Usage Terms, Location fields, or geo-tagging your images, as many of these fields (and others), are not showing up in Photoshop, Adobe Bridge, Photo Mechanic, or Expression Media, after being exported from Aperture 3.
Visit the "Apple Aperture 3 Metadata Issues" article to find out more, including access to a chart that maps out what is seen and what is lost or hidden in JPEG, TIFF, DNG and proprietary NEF files.