I'd start looking into implementing
reCAPTCHA. Either that, or
KittenAuth. However, reCAPTCHA has the added benefit of actually helping to digitize books.
* Edit: Taking my own advice, I went ahead and did a little reading up on the reCAPTCHA API. Suprisingly, I've gotten it working with relative ease thus replacing the default CAPTCHA on registration. Expect a release soon.