Imagine you are switching from an media management application such as iView Media Pro or even Photoshop (on Windows or MacOS), where you have been carefully tagging your images with IPTC keywords, to iPhoto. You will be shocked and dismayed that you spent all that time tagging, only to find that iPhoto doesn't understand IPTC keywords, and will not display them alongside your images when you import them into your library.
The reason for this is that iPhoto uses its own method of storing keywords seperate from the image file in the library. IPTC keywords, on the other hand, are metadata stored in the image file itself. I was in that situation myself a few months back, so I went and developed a solution. I have written a couple of Applescripts that allow you to import IPTC keywords to, or export your iPhoto keywords from, the images in your iPhoto library. You can find the scripts (and more info on their dependencies) on this page at scriptbuilders.net.
You use the scripts by selecting the images in iPhoto you want to import/export to/from, and then run the scripts from the Finder or from the Script Menu, if you have installed them there.
Here are some possible uses:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051113185618105