Yep, my site dropped from PR4 to PR2. I didn't lose and spots in the SERPs but I worked hard to get that PR4. I built hundreds of high PR blog comments and Edu & Gov backlinks too. What makes a site lose PR? The only change I made was that we loaded a mobile version of the site and it is getting more traffic than the desktop version. Traffic count does seem to matter for PR. Could the drop in traffic lead to a PR drop?