Hey there, I work for NuCaptcha and have posted your comments on our support form at Get Satisfaction. Our tech team will post a response soon. Thanks for your interest in our product.
Here is the link your question of NuCaptcha plugin not working on 3.1
http://questions.nucaptcha.com/nucaptcha/topics/why_is_the_nucaptcha_plugin_is_not_working_on_wordpress_3_1_sites
