Just to clarify: Piwik is and will remain open source. Piwik PRO is a separate company and has no direct ties with Piwik anymore.
Edit: FAQ Item: http://piwik.org/faq/new-to-piwik/#faq_21984
Edit: FAQ Item: http://piwik.org/faq/new-to-piwik/#faq_21984
Have a look at http://piwik.org/faq/new-to-piwik/#faq_21984 for a more detailed explanation.