Hi Chip, excelent plugin, saves me a lot of headaches.
However, there are some issues I would like you to take into consideration for the next version:
Problem: Line 88: $cbnet_rscc_captcha_image_url = get_bloginfo('wpurl') . '/wp-content/plugins/really-simple-captcha/tmp/';
Solution: Content directory should no be hardcoded. If you are using define('WP_CONTENT_DIR', ); and define('WP_CONTENT_URL', ); in your wp-config.php the CAPTCHA image will not be visible. There are some plugins, e.g. Better WP Security that allows to change this.
The best is to use the function: http://codex.wordpress.org/Function_Reference/plugin_basename
Problem: If you are not using the standard comment_form() function. In 3.5.1 it throughs the error:
undefined function cbnet_comment_captcha() in comments.php
Hope this help.