youfoundjake - 2:30 am on Mar 13, 2011 (gmt 0)
Oddly enough, almost a year ago today, there was an interview with Matt Cutts that asked that very question. He later clarified it in an email, but to quote:
Matt Cutts: That's a good question, and I am not 100 percent sure about the answer. I can certainly see how there could be some loss of PageRank. I am not 100 percent sure whether the crawling and indexing team has implemented that sort of natural PageRank decay, so I will have to go and check on that specific case. (Note: in a follow on email, Matt confirmed that this is in fact the case. There is some loss of PR through a 301).