WARNING: This will slap a copyright credit in your footer without so much as a say so. It also comes with big capitalised warnings about removing the credit in the code.
Include a credit by all means but slapping an uneditable line halfway across your footer on every page is a bit too far - Bang out of order. The WPMU Super Captcha doesn't appear on every page if your using it to secure the signup only.
Hint! - remove the credit from the footer and place it more discretely within the Captcha itself. Don't be so pompous as to screw up someones site and theme style.
To heck with the warning, I've remove it anyway and placed it on the bottom of my sign up page. WHERE IT BELONGS!!