Google's latest PageRank and Backlink update started on June 22, 2004 and had finished propogating across the datacenters on June 23, 2004. While most people reported normal changes, with a rise in backlinks corresponding to a rise in PR, there were some reports (myself included) of large increases in backlinks, with a decline in PR.
I had 3 sites that tripled the backlinks, but the PR on each went down from 6 to 5. Some speculate a reduction of importance in forum backlinks. That would correlate with my 3 sites.
In addition, some sites were penalized for the first time because of their association with some dubious SEO firms. These firms were using onmouseover javascript redirects and the sites were nailed for it with a PR0.
Be careful who you trust and use common sense when allowing any SEO to touch your site. If you have any doubts about the methods they use, do your research. Better to be lower in the rankings than banned completely!