I followed the recaptcha instructions and created a simple app that works if I run it in Eclipse using a local Tomcat server but the same files give an error (verify-params-incorrect) when validating the user's response when run in Eclipse using the local Google App Engine SDK.
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                A: 
                
                
              Are you using the Java code linked to on the reCAPTCHA site? I took a quick look at it, and I suspect that the SimpleHttpLoader class uses some Java classes that are no allowed by Google's AppEngine.
So, here is what I know: Running locally, I get the same error as you. But, when I deploy it to AppEngine, it works! For the forum posts I've read, I suspect it has to do with the remote IP address (127.0.0.1) when running locally.
                  abendigo
                   2009-05-06 04:06:39
                
              Yes I am using the jar recaptcha4j-0.0.7.jar downloaded from: http://code.google.com/p/recaptcha/downloads/list?q=label:java-Latest
                  Jamie
                   2009-05-06 13:56:54
                Hey, I'm still working on it. No luck so far, but I'll let you know what happens.
                  abendigo
                   2009-05-07 01:19:03
                
                
                A: 
                
                
              This is sort of an impertinent answer, but are you sure you really need to pester your potential users with a captcha at all? There's a great article over on sitepoint entitled "10 Things to Check Before Using a CAPTCHA" (http://www.sitepoint.com/blogs/2009/05/14/captcha-alternatives/).
                  Craig Jones
                   2009-10-20 06:28:01
                
              It a good article and point of view definitely worth reading and think about I actually will next time I think about CAPTCHA usage. It just this is not the place... :-).
                  Boris Daich
                   2009-12-08 09:14:28