Did you test it with javascript turned off? I'm not sure if it is due to other customization in my recaptcha setup, but the original code had these lines:

<textarea name="recaptcha_challenge_field" rows="3" cols="40"></textarea>                                                                    
<input type="hidden" name="recaptcha_response_field" value="manual_challenge"/>   

Without them, the user missed a field to post his response in on my site, when not having javascript enabled... The following works for me:
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return '<script type="text/javascript" src="'. $server . '/challenge?k=' . $pubkey . $errorpart . '"></script>
<noscript>
<div>
<object type="text/html" data="http://api.recaptcha.net/noscript?k=' . $pubkey . $errorpart . '" width="500" height="300">
<!--[if IE]>
<iframe width="500" height="300" frameborder="0" src="http://api.recaptcha.net/noscript?k=' . $pubkey . '"></iframe>
< ![endif]-->
</object>
<textarea name="recaptcha_challenge_field" rows="3" cols="40"></textarea>
<input type="hidden" name="recaptcha_response_field" value="manual_challenge"/>
</div>
</noscript>';
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Thanks for the patch though.!

/Torin