The discussion continues on comment spamming and a couple of people have taken my initial quick fix and expanded on it nicely.
Jennifer from Scripty Goddess has taken to solution into the MT tmpl files, adding the hidden field to processing.tmpl.
Brad Choate came up with a fairly complex solution that, while not keeping a determined spammer out, would force the person to work for their spam.
Joni Electric has a good re-cap of effort to date.
(Found through trackback, by the way.)