Hm, my previous post on subject seem to have fallen in /dev/null (can't find it even in list archives, strange).
Anyhow. While on the subject of spam control. I was playing lately with Domain Key (and also newer Domain Key Identified Mail) systems. It's currently primarly used by Yahoo and Google Mail (gmail). For example, all emails from gmail.com on this list have DomainKey-Signature header. Also, all of those emails fail the verification. It is most likely due to the mailing list mangling the Subject line (addtion of [CentOS] tag) and body of the message (addtion of footer).
Currently, Google uses "in testing flag" in the DNS record (as can be seen from output of "dig beta._domainkey.gmail.com txt"), so none of those emails should be rejected by anti-spam software. However, if they change the flags to instruct anti-spam software to reject emails with invalid signatures, the emails from this (and many other) mailing lists will start bouncing big time.
I guess if these systems become widespread in the future, either the mailing list will need to be updated (so that it regenerates signatures with its own key, as defined in proposed standards) or mangling of subject line and the body will have to be abandoned... Maybe it's a good time to start preparing for the possible future.