I've rewritten this post since original publication.
You can trace the evolution through this Apple Discussion thread: When you drop an image into iPhoto in 10.4.11 the Finder metadata (extended attributes – Spotlight comments, Labels, etc are not copied.
I thought this was an iPhoto bug, and I bemoaned the lack of attention to Finder metadata.
Apparently it's an OS bug! In 10.5 metadata, including Spotlight comments, is preserved.
So I can't blame the iPhoto engineers, but that's a pretty amazing OS bug.