Another option you might want to look at is the math captcha module (http://drupal.org/project/captcha), which provides you with the ability to use a match captcha, rather than a graphical one. (So the verification is "What is 5 + 7", rather than ("Please type in the letters/numbers..") I use it on my blog and it works great. From what I understand, math captchas are considered better for accessibility purposes (easier for people with screen readers, I guess.)

Congrats on the new plugin!