We did actually try a system like that at one point, people were asked something very obvious like "what planet do we live on" or "what is three times two"
However the problem is that the system got very intrusive, popping up all of the time even when not wanted or when the member was already established, not just at account creation and when posting the first post as it should've done.
Kingdom of loathing employs that sort of system and it seemed to work well, but unless we can find a reliable PunBB extention that only works once, it probably won't be that useful.
Also however, bare in mind a lot of spambots are actually registered by human operators anyway, even though they don't actually have the human reading the messages, look at all of the world of warcraft spammers we get, so a logical capcha wouldn't discourage that sort of thing. Indeed, I'm not sure how much those idiotic, inaccessible image capchas you see everywhere do either other than be constantly inconvenient.