How do comments work in ZenPhoto? Is it possible to have threaded replies? Will the original poster get notified of a reply?

1. No.
2. Only if he subscribes to the comment RSS feed.

Is this something that might be added at a later date, or is ZenPhoto built in such a way to allow plugins like Subscribe2? http://subscribe2.wordpress.com/

There is actally already a plugin involved in comment handling--the spam filter. You could create a spam filter that sends replys to other commenters.

You can use free comment system from http://js-kit.com

Can be customized totally using css

sbillard; how would that work? Just place Subscribe2 in the zp-core/plugin folder?
I also noticed there's a plugin already in there called "image rating".
How does that work.

farhan; that's pretty cool, but it also makes me a little uneasy. I'd want the comments stored on my own server, not someone else's.

No, won't work, this is not Wordpress. Only the spam filters are plugins (special zp plugins), the other files are not.
The rating functions are normal template functions (as is the printalbummenu, but separated). The rating folder contains the JS and uupdate stuff that needed to be stored separate from the template functions. We just didn't want to have one more folder within zp-core, so we choose the plugins folder.

No, you would have to actually make the plugin work as a sync between Subscribe2 and Zenphoto. You cannot just place files in the plugins folder from other systems and hope it works!

In order to use ratings:
in image.php
<?php printImageRating(); ?>

in album.php
<?php printAlbumRating(); ?>

Gotcha. That's what I figured.
Say for example I was able to get someone who would write a plugin similar to subscribe to... is there documentation that they could read to get it to sync with ZenPhoto? Or are the source files enough?

There are several spam filters you could look at for examples of how to write one. The simplest is none.php.