After the installation, you get a small (import hCard) link behind your URL field in the comment form (if not, use
<?php hcard_commenting_link() ?>).
If you want to write a comment using this festure, you simply have to fill out the „Website“ field with an URL to one of your hCards and to klick the „import hCard“ link.
…and thats it.
To try out this plugin, you can use my comment form.
One of the next steps would be some kind of avatar feature using the hCard
photo (thanks to Brian Suda for the idea).
You can download this version on the WordPress Plugin area: http://wordpress.org/extend/plugins/hcard-commenting/