What about but "questions" are not ("Result for 3+5" + textbox, "What's your favourite scripting language?" + textbox) instead of captchas?
That's the sort of thing I had in mind. Technically, they are still captchas, just text-based rather than image-based. We use them on the IPython wiki (http://wiki.ipython.org/Main_Page ), and while it doesn't stop all spam, it cuts it to a manageable level.