Like I say in the privacy statement, I only log anonymized info.
"I anonymize the last byte of your IP address with the piwik AnonymizeIP plugin. That means that rather than your IP showing as 126.96.36.199, I would see 188.8.131.52 . "
This gets aggregated monthly which ought to make it more difficult to deanonymize a visit. Be interested to hear if other people can suggest better ways to achieve this.