I don't think that is the main cause of the change.. Most probably it is due to the drop in backlinks quality..
Only IP cause to PR might be c-class but it's not really probable if you have fair amount of links, update is not on yet anyway, maybe your PR was somehow faked so it looks dropped now.
google didn't even changed the PR lately. Please check your PR with another tool. Or, maybe it was fake.
Change your ip address will not effect your site's pr.I own a site with a low pr,i redirect my site to another address,and i have my site's pr remained.
Hey guys! I switched hosts on my fan-site a few weeks ago, and it changed, so that may be the issue with you.