I might be behind the times, but for people stuck with iPhoto 6, here’s a quick guide to getting your photo titles, keywords, comments, and a copyright notice into the EXIF header of your image files.
The AppleScript that works for me is from Andrew Turner. I modified it as described in this comment to enable apostrophes in keywords. In addition, my installation of exiftool from Fink only works from AppleScript when giving the complete path and the location of its libraries, like this:
PERL5LIB=/sw/lib/perl5 /sw/bin/exiftool. Also, don't forget to change the copyright notice at the top.
Save the script in ~/Library/Scripts and make it available by checking "Show Script menu in menu bar" in AppleScript Utility. Now you should be able to select photos in iPhoto and run "Set Exif Data" from the AppleScript menu in the menu bar on them.
By the way, if you're stuck with iPhoto 6, you almost certainly want Ken Ferry's excellent Keyword Assistant for iPhoto.