ProBlogger’s page rank is currently at a 4...! Have you observed this? Check it out for further information ... http://www.problogger.net/archives/2007/10/24/problogger-pagerank-4/
Not really, PR to me is just a number. If your traffic doesn't decrease then you really should care. I don't judge the sites I go to by their pagerank.
I could be wrong, but I think if his SERPS are affected by the PR drop it will probably hurt him less than most bloggers, because there are so many ways to find his blog other than Google search with all the blogs linking to him. PR4 is still ridiculous though for a blog with as many backlinks as he has. Even though he sold links for awhile his blog authority is still higher than most PR4 blogs, so its really going against what their algorithm is all about.
Darren has over 34,000 RSS subscribers and over 13,000 links in Google, I think he will be just fine.