I have seen the question asked many times so thought I would share something I just found this on a drop list site. (Not that it's a definitive answer.)
"May 7, 2004 : You may notice that Google is getting better at deleting expired domain Page Rank. Domains in Pending Delete are getting hit a lot more than any others. As of right now, they are running 4 days. What that means is when your domain is 4 days away from expiring...
PR will be dropped. In the past,
PR restored quickly with a content rich site. This may or may not be the case now."
Have you had any experience or read any other info about expired domains and Google
PR?