Maybe Google understood that you over optimized your site, and maybe they thought you built a site for the search engines and not the consumers. Just my 50 cents!
I've run across something similar for my network. The very first blog I ever made and the one that brings in the most traffic was a PR2 for the longest time and is now a PR3 - probably because I recently started getting rid of a bunch of sitewide outbound links I had through old trades. My second stayed at PR2, unless the update isn't done yet. I'm thinking it's my outbound links. Another blog went from PR3 to PR4 and that one has minimum outbounds. A newer one I made back in February jumped up to PR4 already and doesn't have anywhere near the inbound links my first two do, but hardly any outbound links. It's also ranked number two on google for it's particular keyphrase (semi-competitive, the #1 is the site I'm marketing for). I've never been high on PR but I've learned a little something useful from this update. Meanwhile the 12 or so other blogs I made in late February and March haven't ranked at all, so I'm guessing they missed the time window.
Penalties last week - but with the new update things are all up - Id' say we are coming through the woods steadily boys..
+1 Post count. Congrats. No its not true. Same penalized basis as all PR updates. It is still going on though, yesterday my blog was PR3, today its PR6.
my page rank for ISRdesign.com is 1 and for http://www.tutorialephotoshop.isrdesign.com/ ( i have redirect from ISRd to tutesps.isrd...)