I have the same since december my traffic from Google for my main site went from 400+ to 5 - 10 visitors. I started the site in july and since the last update i still dont have any PR for my website. I have link 171 back links and 220 pages indexed by google. This not only for my main site but also my other sites what i started since august-oktober dont have any PR. I am not sure what is going on. Need to wait and see.
The PR update should not affect the already incorporated SERP as mentioned by Matt Cutts in his blog. I would say that the main issue that causes this is the regular data refresh that Google does. This often happens as we saw some of these threads popping out from time to time..
True, but this is the worst I have ever seen, as far as a refresh goes. To lose anywhere from 40%-90% of Google referrers in one day is ridiculous.