There are some interesting tidbits on the spam filtering in GMail in the paper "Sender Reputation in a Large Webmail Service" (PDF) by Googler Bradley Taylor.
In particular, it sounds like one of several spam filters in GMail uses the aggregate data on what data is marked as spam (by other automatic classifiers or manually by the user) to determine the spam reputation of a domain.
If e-mails sent from a particular domain are consistently marked as spam, the reputation of that domain will deteriorate, and then eventually the entire domain will be blacklisted. Simple but good idea.
I'd really like to see an extension of this where reputation of individual senders are monitored, not to blacklist, but to whitelist.
For example, Findory has a feature where people can get a daily e-mail of their Findory front page, essentially a mailing list. As the Google paper says, "Some users are lazy and find that reporting spam on a mailing list is easier than unsubscribing ... For the sender it means their reputation is hurt worse than it needs to be."
However, mail from email@example.com presumably is never marked as spam. Whitelisting individual senders with good reputation seems like it would be a good refinement to blacklisting domains with bad reputation.