That's strange. Have you tried entering a title or description in the zenphoto data for the image? If the function can't show IPTC, it will fall back on whatever is stored in the database, so you should not be getting a blank.
It could be that your Canon stores the tags differently than my Nikon, and the IPTC parser in PHP doesn't know how to extract them... I have to confess I have no idea how portable the $iptcTags array is, or if there's a better way of getting at the tag data. I'll try and chase down some documentation on the iptc parser.