After a fairly long and frustrating day, I finally have DKIM working with Postfix on Etch. I was assuming it would be fairly straighforward, but was treated to such lovely errors as these in my mail.log:

smtpd: warning: milter [...] can't read SMFIC_OPTNEG reply packet header: Connection timed out
smtpd: warning: milter [...] read error in initial handshake
cleanup: warning: milter [...] can't read SMFIC_EOH reply packet header: Success
smtpd: warning: milter [...] can't read SMFIC_MAIL reply packet header: Success
smtpd: warning: milter [...] can't read SMFIC_OPTNEG reply packet header: Connection timed out
smtpd: warning: milter [...] read error in initial handshake