Okay, adding comments lead exactly to the expected situation of spam comments, so I just added fccv and started using typepad to help me in fighting with spam. I hope it will catch most of it (it would catch all I've had to delete manually so far) and will avoid false positives.
For some time (probably since upgrade to 1.0) it looks like my Piwik installation did not log URLs and page titles from visitors. Simply all URLs and page titles logged were blank.
While I still postponed the investigation of this, I was pretty sure it must be some configuration issue and not a bug, so I felt reluctant to report this.
Today I finally found time and copied upstream
global.ini.php to my installation and it seems to do the trick. I should better care of upgrading next time :-).
PS: Any volunteer for packaging piwik for Debian?