I have tested having both WP-Hashcash and Bad Behavior enabled.
While I don't know what Bad Behavior has been doing in the background, I have to say that I really like WP-Hashcash, and the fact that is sends potential spam in moderation. At least, I had a sign of life from this plugin, it works!
In theory, I like the idea that Bad Behavior stops most bots at the gate, so you save on bandwidth (which you are paying for). This is not just preventing bots from posting a comment, but from reading your site in the first place.
Now, one has to trust that this plugin is extremely reliable with the "good" bots, or else goodbye site ranking.. Because it acts in the background, how does one know for sure?
With that said, ljmac has a point, so I will now try enabling Cookies for Comments, in combination with WP-Hashcash.
Edit: In all fairness to Bad Behavior, there are logs that can be accessed through phpMyAdmin.