Marketing_Guy - 2:19 pm on Mar 12, 2012 (gmt 0)
It's not actually necessary for Google to keep track of these old domains for 301s to work. Theoretically, the redirection is all done by the server and browser/user-agent.
True, but wasn't it the case for a long time that spammers would buy up expired domains and throw some content on them based on previous subject area to leverage the old domain's "SEO history" (for want of a better term) within that niche?
That's a separate metric from PageRank (essentially qualitative vs PR's quantitative nature). It would be possible for Google to maintain the history of a domain (in terms of subject area) quite easily and this would be useful to flag sites which change dramatically.
Looking at the precise statment from Google;
We often use characteristics of links to help us figure out the topic of a linked page.
If there is a qualitative metric associated with a 301 redirect (i.e. Google uses it to pass the subject area from old page to new page), Google could switch that off with minimal ranking distruptions for legitimate use (i.e. PR is still passed, or at least a percentage of it). New page would still rank as well as it otherwise would (it's optimised and is receiving most of the old PR), but completely changed pages wouldn't necessarily be able to achieve the rankings of the old URLs (they are judged on how they are now rather than how they were).
May muck around with a test on this one when I get some time.