It's probably a penalty of some sort. There's long been speculation that Google considers DP a trashy site. http://www.seoco.co.uk/blog/more-proof-that-google-loves-digitalpointcom/ Strange because DP does well enough in the serps. Actually I get the sense that DP has improved a lot in the serps lately, so my guess is that the PR will bounce back to 6 or 7 in the next update, or the one after that.
The forum PR drop to PR4 since the last update. If you remember alot of sites and blogs also drop in the last update. I think it is as a result of change in Google as well.
Not only answered above but this thread has gone way off course. the questions that were asked were 1. Is there a pr Update going on? Still not answered? 2. When did DP's page rank changed. Question answered above. No one asked anything about pr value or anything to the effect. It was a simple when did it change. In other words was it in the last day or so?
I think that Google does not like DP members much especially after many started to bash Google for the wrong things they've done to them and their AdSense and AdWords accounts. Google will raise the PR after people here start saying only nice things about Google. In all seriousness, I think that the PR dropped because there're way too many people linking to their sites in their signatures and there are not as many inbound links. Also, linking to sites that Google banned or does not consider of any value lowers your PR.