Eoin,
Sorry for the delay in getting back to this. We've tested a bit and can't recreate your problem. However, we'll explain our testing config and point out a few possible problem areas that you may be running into.
First off, captcha was first included in Joomla with version 1.7.3 (I believe). However, the first few revs of it was pretty buggy. The captcha that would be showing up on your registration is the integrated Joomla captcha, not the one built into RSForums (you mention this in your private message). So, we'll focus on the Joomla captcha.
If you're still using Joomla 1.7.x, I'd highly recommend upgrading. In addition to numerous security fixes, the captcha system has been improved. If you're using an earlier version of Joomla 2.5.x (up to 2.5.3, actually) and you had upgraded from 1.7.x, there was a separate bug regarding captcha when you upgraded between those revs. Again, I'd recommend going to 2.5.6 which should fix those problems.
Next, let's check your captcha config itself. If you want the captcha on registration, you should:
* Go to your Plugin Manager. Look for "Captcha - ReCaptcha". Edit it and make sure you've entered a Public and Private key.
* Go to Site -> Global Configuration. On the Site tab, for Default Captcha, set that to "Captcha - ReCaptcha"
* Go to Users -> User Manager. Click "Options"./ For Captcha, set it to "Use Default" or to "Captcha - ReCaptcha" (we used default for our testing, but both should have the same effect).
For testing only, set the Error Reporting in Joomla to 'maximum'. That will help locate any issues below easier.
To test, I'd recommend trying to go through the normal Joomla registration. You should see the captcha form. Test filling it out incorrectly to make sure you get a "Not allowed" message and then try correctly to make sure your account was created. If that doesn't work, you're probably affected by one of the bugs mentioned above. If you're on Joomla 2.5.6, let us know, and we'll try to help you debug that. If not, our only recommendation would be to upgrade.
Then, if normal Joomla registration works, then try the JLinked registration and see how that goes. Test the same way with a 'bad' captcha and a good one to see if everything works right. You shouldn't see any errors or unexpected problems.
Finally, set Error Reporting back to 'None' (the recommended value for a live site) and let us know how it all went. The captcha box should work with our registration flow, so if it's not, something is wrong, and we'll help you get it going!
Good luck,
Alex