Currently it supports automated message archiving and message retrieval through XEP-0136 compliant Jabber clients and a web UI for the administrator. Support for manual archiving and preferences is planned for future releases.
Version 1.0.1 requires either MySQL or the embedded HSQLDB, support for other databases might be added at a later point.
To install the plugin just rename the jar to archive.jar, place it into the plugins folder of your Openfire server and enable it in the Openfire console at Server Settings/Archive Settings.
For those interested in the source code have a look at its development site.
Update June 27, 2007:
Now we also have some screenshots available: