Such an integration could range from very simple to very complex depending on the developper time & passion.
A few ideas
You could have a look at !http://wordpress.org/extend/plugins/phpmyvisites/ -- a phpMyVisites plugin for WP compatible with Piwik, just to start with. I haven't tried it, but maybe it's useful for your work.
What's the thinking here guys?
Do you see value in Piwik developing a collection of plugins for popular CMS platforms (eg: WP, Drupal, Joomla) or do we "relinquish" this task/effort to individual CMS communities (eg: WP plugin developers)? - thus freeing us up to focus on Piwik even better ? Or do you see the inclusion of these add-ons as part of the Piwik package and value add?
Eg: three plugins that add Piwik integration to Wordpress and work well with current WP 2.9.2 and Piwik 0.5.4:
Remember that Piwik already has the nifty feature that allows users to generate necessary code/script to drop a pretty Piwik widget anywhere on one's site.
Suppose we decide to "relinquish" development of certain add-ons to external communities - we could:
1) "Interact/Collaborate" with these external communities (eg: via WP plugin forums) thus strengthening and expanding the Piwik network even further.
2) Acknowledge the external contributors via Piwik Blog posts / Tweets etc...
Robert, I think we just want to help plugin authors building such features if they want to. We don't want to take over wordpress integration plugin for now, as already there are a few plugins that you mentioned that are doing a nice job. I mark this one as fixed for now.
See also the blog post on piwik.org http://piwik.org/blog/2008/06/new-plugin-wordpress-piwik-integration/