Firing up the CouchDb server on Windows is a breeze—follow the README. PHP-wise, you need the new http extension which is most easily done on Win32 by grabbing the most recent PHP 5 release (5.1.6) and the corresponding collection of PECL modules.
The interface between CouchDb and PHP is REST - XML + HTTP - you can also point your browser directly at the CouchDb server (default - localhost:8080) and get around with a little help from the CouchDb wiki.
So, if it's Just Another Database, why should we pay attention? Harry notes (with a code example) that it's more about how it stores the information and not just that it does. His example takes in a POST request from a form and pushes it (raw data) into the CouchDb functions. He also gives an example of where this would be handy - in a wiki (like Dokuwiki) where the files are currently stored on the filesystem instead of in a database.