tedster - 5:56 pm on Feb 3, 2011 (gmt 0)
One of the challenges is that Google is (and needs to be) complicated about how they deal with 301 redirects. They used to be a spammer haven, so all kinds of trust checking needs to occur.
Also technical errors are extremely common, so they can't just abandon a URL once they've verified that a 301 is in place - the website may easily change their mind. In other words, a "permanent" redirect is not truly permanent in the practical world of today's web.
So Google does have a challenge with 301s - and as long as the legacy URL is only showing in the site: operator results and not sending actual search traffic because it is the version that is ranking.