Two possible methods were suggested -- mark hunte recommended the first is a Unix program called rdjpgcom. This app can be installed via Fink, or you can install it yourself from source -- I found this page which provides a few helpful install instructions. Once installed, use man rdjpgcom for information on how to use the application.
The other method is an AppleScript, written by robJ, that creates a text file containing the path to matching files, and the contents of the matching files' comment fields. Check out the forum thread for the script's source...
I haven't directly tested either of these, but thought it might be helpful to some folks out there.

