When the option to dissalow multiple accounts on one email is set, it'd be cool if (like the username field) the ajax registration verifier would check if the entered email address is already being used on an account.
This is already implemented. You'll find the setting in User Profile & Registration Options.
The option to allow one account per email address is included. However, having the AJAX on the registration page check to see if that email is already in use is not part of MyBB yet. I think this is a valid suggestion, don't you agree?