We have been using Piwik for 3 months now on our website, Scientific Blogging.
For every one of our writers, a Piwik site and user is created automatically. Every writer gets a Piwik widget on their dashboard:
http://img403.imageshack.us/img403/1346/picture4hx7.png
and a link to Piwik statistics for t heir own articles, which takes them to an Iframe that logs them in automatically to their own Piwik site: