I am pretty sure thats its done quarterly, i can remember reading about this somewhere on http://www.mattcutts.com/blog/.
This update the PR for my site went from 3 to 0. My site is http://www.power-battery.com.au and is focused on freebies. I do alot of site-wide link exchanges with other freebies, free fonts, webmaster resources, and freeware sites(basically alot of sites with free offers). Could this possibly hurt my ranking. I was just wondering why the sudden drop in PR and if it could mean a penalization.