While it is common to have music metadata stored inside mp3 or ogg files, most photo management applications (e.g. gThumb, F-Spot, Picasa) still use their own database to store the tags. As a consequence it is easy to switch between different music players, but almost impossible to change photo management application once you have tagged your photo collection.

This page
http://f-spot.org/To_Do
shows that F-Spot developers have a plan to implement the feature to be able to store all metadata in a XMP file in the JPEG.

I think this feature would be really important for gThumb too, so that users are not tied to a particular application. This is even more relevant when one thinks that gThumb is not available for Windows, and many people want to be able to use (at least occasionally) that OS.

By the way, I really like gThumb, and I don't mean I want to change application, but I'd appreciate to be able to try out other programs just to have a look.