Spotted a rather snazzy replacement CAPTCHA plugin today called "Are you a human". In effect it gets the user to play a mini flash/HTML5 game rather than the traditional work out the distorted characters (which is a PITA ofc).
Their site is at http://www.areyouahuman.com/ and the WP plugin is at http://wordpress.org/extend/plugins/are-you-a-human/
Well, I'm not sure what it is tbh.
1. Turned off the old CAPTCHA plugin ("SI CAPTCHA Anti-Spam" by Mike Challis... pretty good one) and turned on "Are you a Human"... no luck
2. Turned off the first commentators workflow I set up (which is "Comment Redirect by Yoast" and "Comment E-Mail Verification".. basically it redirects new commentators to an instructions page and also sends them an e-mail with a validation url).
3. Cleared the Page & Minify caches of "W3 Total Cache"
4. Forced a clean reload of the page in my own browser
The best I get is
a. no flash/html5 mini-game on the comments page
b. when the comment is submitted it shows a "We could not verify you as human. Press your browser's back button and try again." error page (standard WP error page that has been customised by the plugin I believe).
I don't run many plugins, and no others look likely to cause interference.
I don't know it is the Suffusion Theme, but it's the next most likely candidate I think.
I don't want to turn Suffusion off, even for a test if I can avoid it so thought I'd post first.
Here's hoping someone can try this relatively new plugin with their own Suffusion site and assist!