Hannes this is fantastic,

I would love to see in future updates the ability to put a simple field which requires the user to enter a coupon or registration code, which has been predetermined and even hard coded into the php file rather than a full backend module.

Even something as simple as:
-User signs up with your autoresponder
-User is sent 1st email which has the registration code and link to the registration page
-User enters code (along with usual info)
-If correct user gets access (to registered member content)
-If incorrect user gets sent to a page stating 'incorrect code' with a link back to try again

This would be almost the same as your Terms of Use setup but with a text field instead of a radio button field.
I'm trying to do it myself but I'm a real php noob.

Great add-on so far, will definately be adding it to all my sites.

Cheers