my clients don't want to use Akismet (for whatever reason clients have sometimes ;-), so I implemented a simple honeypot trap. The idea is you add a "text" field with id="honeypot" to the form, and a new contact_form_is_spam filter rejects the form if there is any content in this field. In this way, the use of a honeypot field is optional and per-form. A little CSS to display:none the honeypot field and voila.
Works nicely and traps 95% of all automated (bot) spam coming through the form. But can't be hooked in without a few changes to the Grunion plugin.
Wondering if there's any interest by the developers to incorporate this functionality - I'm happy to submit a patch if so.