I am looking into this as well, I was hunting around and found several PR4 domains so I grabbed them at the normal domain costs, I am interested in seeing the results of PR change... I have added the links to multiple sites with PR2, 3 and 4, I am wonderring if it's to late to resurrect them and if there's a certain period of time that you have to act.
I found http://www.mikeindustries.com/blog/archive/2005/03/how-to-snatch-an-expiring-domain a pretty good read on expired domains and some of the main companies that grab them. It sounds to me like it's pretty hit and miss, and some of the companies business practices are a bit dubious.