Whitelisting on the subject line?

Have you thought about doing any whitelisting on the subject
line? (You can read "whitelist" as any color you like--
I'm talking about what David Tamkin calls a "greenlist".
I hope we don't end up with too many colors to keep track of.)

For example, since you have a website about procmail, you might
want to accept anything with "procmail" in the subject line.
You might pick up a few random-word spams here and there, but
probably not much. And it would decrease the number of false
positives because your spam filters would never see them.

The idea is that if someone is sending you (real) email, and
they're not in your whitelist, then they must have gotten your
address from somewhere, and that's probably reflected in the
subject line.

What concerns me is that the spammers are using very generic
subject lines these days, so even if a message passes all the
spam filters, I might not bother to read it if the subject line
doesn't say anything specific and I don't recognize the sender's
name. People are going to have to learn to write meaningful
subject lines.