If you use iPhoto, but keep the original images external to the iPhoto Library package, this hint explains how to move those externally-referenced files to a new location and update the iPhoto library so that iPhoto knows about the move.
This hint is for iPhoto 6 and higher.
To update your Library with the new location of your files, you need to go into the Package Contents of you iPhoto Library (Control-click on iPhoto and pick Show Package Contents from the pop-up menu]). Once the new folder opens, navigate into the Originals directory. There you will have a directory for each year of which you have photos.
If you have externally-referenced images, then the images inside of the Originals directory structure will be symbolic links to original files. Update these directories with symbolic links that point to the new location of your files. For instance, for my 2008 library, I renamed the 2008 directory in Originals to 2008_, then made a symbolic link to the new location of the 2008 folder, and put the link in the Originals directory and named it 2008.
Now when iPhoto goes looking for the original photos, it will find them via the symbolic link to the new location. Don't mess with the Data and the Modified directories, since they contain local images like thumbnails and modified files.
[robg adds: I can't test this one, as my photos are all stored within the iPhoto library.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090722163512346