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:
- Migrating from another OS or media management application to iPhoto.
- Exporting you iPhoto library for backup purposes. Now your keywords can be backed up, too.
- Uploading images to Flickr. Flickr understands IPTC keywords in your images, and automatically adds them as tags.

