By the same token, the point was never to be able to spot spammers by noting who isn't using SPF. Rather, the point is to make the blacklists more reliable. It is *only* when you use SPF in *conjunction* with blacklists/whitelists that you see any benefit from SPF.
It seems to me that the point is to make whitelists (aka greenlists) -- not blacklists -- more reliable. If a MAIL FROM is in a blacklist, the spammers will know that and use a different MAIL FROM so blacklisting will not be helped much by SPF. But if a MAIL FROM is in a whitelist, it will get tagged as non-spam (by, for example, my procmail recipes) and it will be hard for a spammer to pretend her MAIL FROM is in my whitelist because she cannot send through the SPF-approved SMTP server for that MAIL FROM.
I've written some about this on my Procmail Quick Start in this section:
<http://www.ii.com/internet/robots/procmail/qs/#greenlisting>
I hope this make sense, NM
-- Infinite Ink: <http://www.ii.com/> Reverse Spam Filtering: <http://www.ii.com/internet/messaging/spam/> Procmail Quick Start: <http://www.ii.com/internet/robots/procmail/qs/> IMAP Service Providers: <http://www.ii.com/internet/messaging/imap/isps/>
