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:

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.

Forum discussion at WebmasterWorld.