Maybe it would be helpful to track users without using cookie and/or IP addresses:
have a look at (german): http://www.golem.de/news/tracking-der-verraeterische-fingerabdruck-des-browsers-1212-96167.html
Basically the same idea as: https://panopticlick.eff.org/
I think it might be an interesting idea to implement this, especially with the new very restrictive laws in the EU for Cookies.
Piwik already has such algorithm in place, using heuristic based on resolution, browser, etc. (but also using IP address). if you have more specific improvement comment here. Otherwise we dont plan to change this yet.