application.js

$.fn.transitionToNext = function() {
  ({
    entry: function() {
      // do whatever is needed to transition to entry
      $(this).data('state.next', 'captcha');
      return false;
    },
 
    captcha: function() {
      $(this).removeClass('entry').addClass('captcha');
      $(this).data('state.next', 'fooblig');
      return false;
    },
 
    final: function() {
      $(this).disable();
      // not returning false, so the form should submit.
    }
  })[$(this).data('state.next')].call(this);
}
 
$('#new_user_form form').data('state.next', 'entry').bind('submit', function() {
  $(this).transitionToNext();
});