Am I right to notice that Zenphoto will not import IPTC metadata, like tags, when embedded in a PNG file? If so, why isn't this possible? What's the difference with importing the data from jpeg (which works fine)?

I store all my photos in PNG, since it's lossless and harddisks are dirt cheap. And I was planning to use Zenphoto on an intranet, so bandwidth really isn't an issue.

It should work with .png files as far as I know. The best way to find out is to try it.

Ok, so I just figured out that the problem isn't with Zenphoto, but with limitations of the PNG standard. Apparently there is no official support for IPTC or EXIF metadata in PNG, so these aren't even written to the file in Photoshop/Bridge in the first place. See:
http://heim.ifi.uio.no/~gisle/blog/?p=3

Instead PNG does support XMP, but I guess I'll have to try that out in Zenphoto by enabling the XMP add-on. I'll report back with my findings.

Alright, that seems to work: XMP in PNG. I did need to change the add-on settings for xmpMetadata in Zenphoto to read the metadata directly from the file, instead of the "sidecar" file. But it works fine now, so problem solved!