Slightly off topic, but does anyone know of a server-side implementation of the same concept? I've been looking for a good one.
It's a Python server side client for the cheap-but-paid PIFilter web service.
and there are lot of research papers if you are interested in implementing them : http://portal.acm.org/citation.cfm?id=1021261 http://www.springerlink.com/content/e774824834226tv2