IPTC tags are not covered by the Exifixer library I don't think. So that would entail including IPTC support in a separate library. I think I'll defer to Tristan's recommendation on this one, but my gut feeling would be that while it's not out of bounds to include it at some point, I don't know how feasible it is for the 1.1 release to have it done. IPTC tags are generally used after the photo is taken in most cases (as I understand it) and are supported in photo editing applications such as Photoshop.
So maybe after EXIF is working and the theming portions done, a library can be found to integrate into Zenphoto that supports IPTC.
And not at all, this is a fine thread to ask. EXIF and IPTC are usually found together, but usually EXIF is the tagging done during the photo shooting, and the IPTC usually after photo editing (at least generally the way I've seen it done).