This bug has long been fixed so this patch is no longer required.

Hi Everybody,

We realized just yesterday that the Couch input tag of type captcha
<cms:input type='captcha' />
was not displaying the captcha image at all! It was a side-effect of some changes we brought about in the latest version of Couch.

The bug has now been rectified although the modifications will ship only with the upcoming release.
For now, I am attaching the patch. Anyone having trouble with the captcha please overwrite the original captcha.php with the one attached with this post.