Eric Enge interviewed Matt Cutts of Google and uncovered a few interesting tidbits about Google, but the most significant piece of information was that 301 redirects do not fully pass the PageRank from the redirect. Let me quote you the interview from that piece:
Eric Enge: Let’s say you move from one domain to another and you write yourself a nice little statement that basically instructs the search engine and, any user agent on how to remap from one domain to the other. In a scenario like this, is there some loss in PageRank that can take place simply because the user who originally implemented a link to the site didn't link to it on the new domain?
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).
Whenever you move a URL or a domain name, we are told by Google to use a 301 redirect, if we cannot keep the existing domain or URL. We were also told that a 301 redirect would pass the PageRank. But that is not fully true. As you can see from the interview above. Some of the PageRank from the redirected URL will be lost. How much exactly? That is hard to say but some is lost.
I personally never like changing URLs but sometimes you don't have a choice.