jdMorgan - 2:35 pm on Mar 15, 2010 (gmt 0)
It's impossible for us to know G's algorithm, but the most straightforward line of logical thought would indicate that if 301 redirects now lose some fraction of the original URL's PR, then yes, you'd lose a little PR redirecting example.com to www.example.com.
But you'd be losing that fraction of PR instead of losing all of the PR that was previously assigned to the "wrong domain." And further, you would still be taking the remaining PR (that part which is not lost due to Google's newly-announced discounting) away from that "wrong domain" and giving it to the "right domain," thereby still making it less likely that that "wrong domain" would compete with your "right domain" in the SERPs.
Just a personal opinion, but I think it's obviously still well worth doing. And Google continues to recommend doing it.
The bottom line on all of this is that it's best to run a tight ship, so that *zero* external redirects are required for your site to function, to never change your domain name unless forced to by litigation, to ensure (by design and testing) that from the moment it first goes live, your site can be accessed by one and only one variant of your domain name (e.g. www- or non-www, not both), and to never change your URLs, even if you change your site's underlying file structure or technology.
This change only re-emphasizes the value of planning ahead and of designing your site's URL-architecture, rather than just assigning URLs in a cavalier manner, as many Webmasters do.