I wanted to use the IPTC caption field as a comment in iPhoto, so I wrote this AppleScript. Copy and paste it into Script Editor, and save it as a script. (You'll also need to have GraphicConverter installed.)
Select the images for which you want to have the IPTC caption tag converted to an iPhoto comment, then run the script. Select how you want to log errors (with a special keyword, or in an error log file).
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070216050559834