This is a verry simple captcha challenge. It’s based on the algorithm of Scott Allen (http://www.hybrid6.com/webgeek/plugins/wp-spamfree), a captcha free plugin for WordPress.
The user has to do no other input, just it’s browser should support javascript and cookies. (~95% of internet users have this). So might stop annoying 95% of your users and advice the other 5% to install javascript or to allow cookies support for their browser.
The process is very simple. This module requires CAPTCHA Module (http://drupal.org/project/captcha).
After installing both modules, you simply goto configuration page of CAPTCHA Module admin/user/captcha and chose default challenge type to: Invisible CAPTCHA (from module invisible_captcha).
And that’s it. More information on CAPTCHA module here.
Later edit: Updated to comply with user permisions.