How to disable and grey out a form button if the textfield is empty?
Obviously you will want to do this with some jQuery...And of course there is more than one way to do this, but:

$(function(){
      //Set button disabled
      $("input[type='submit']").attr("disabled", "disabled");

      //Append a change event listener to your input
      $("input[name='nickname']").change(function(){

            if($(this).val().length &gt; 0) {
              $("input[type='submit']").removeAttr("disabled");  
            }                            
      });

      // You could then trigger the a change event on load just to check
      $("input[name='nickname'").trigger('change');
});


That should be the gist of it.  Of course you would want to change your "name" value to whatever your input name is.