RE: IPTC
There are a few issues there.
We're looking for a way for small (read underfunded with volunteers with zero to minimal computer skills) local historical groups to be able to catalog and digitize their collections and make them accessible to the public and the worldwide scholarly community. Coppermine would be ideal for this because it's free, photo and data entry is easy, and it will run on really cheap shared hosting.
Using IPTC would mean adding a layer of complexity and expense. The data would have to be attached to each image with a program capable of doing so, and volunteers would have to learn an additional procedure. The fewer steps for the volunteers and the less they have to learn, the better.
The biggest issue is probably that IPTC is a standard for press images and the metadata fields reflect this. We're going to be cataloging documents and objects in addition to photos. I realize we could fudge things a little, but this would mean getting volunteers to understand that IPTC metadata field "X" is really Dublin Core metadata field "Y." Trust me, this is a recipe for data entry disaster.
My bet is that if additional metadata fields could be easily added to coppermine you would get quite a following in the museum and library communities. The one decent web gallery product that allows for Dublin Core is ridiculously expensive and charges rip-off hosting and maintenance rates.