As far as I know, the minimum possible karma is zero; scores below that are, IIRC, displayed as zero.
Scores in the negative are kept track of despite not being displayed, however. In particular, people with negative karma have a commenting frequency limit, whereas people with zero karma do not.
http://vimeo.com/22099396
What do people think of this, from a Bayesian perspective?
It is a talk given to the Oxford Transhumanists. Their previous speaker was Eliezer Yudkowsky. Audio version and past talks here: http://groupspaces.com/oxfordtranshumanists/pages/past-talks