I have the same issue. I entered the data including reCAPTCHA code, and when I click on send the message is “The reCAPTCHA was incorrect.”. All times, but all times (for now 15) the email was sent.
I have no clue what the error is. I had first the normal reCAPTCHA Form plugin, with the same error. Then I tried the Form Manager, also with the error. So I deleted the reCAPTCHA Form plugin but the error is still there.
I changed also the reCAPTCHA keys, without an effect. I use WordPress 3.1.2. I use the form only as contact form on one site.
The form worked in the past with the reCAPTCHA Form, I don’t know since when the error occurs.