g1smd - 8:49 pm on Jul 14, 2012 (gmt 0)

At SMX London in May, Pierre Far confirmed that URLs returning '410 Gone' are assigned a lower crawl priority than those returning '404 Not Found' and that all such URLs are recrawled on an occasional basis essentially forever. He also said that a surprisingly large number of URLs returning '410 Gone' do eventually start serving real content again some time later; often months or even years later. This is one reason why Google "never forgets a URL".