I'd like to see more color variation in the output to assist those who are red-color-blind (among others) in seeing a captcha after refreshing a possibly bad one they cannot see clearly.
Also Mike, you might consider incorporating reCaptcha, since both other plugins here on WP site seem to have ceased development.
I've used your si-contact-form plugin before and it was nice to find this as well, when I needed it. most of the other captcha choices were abysmal. :D