function submit_signup()  {
	var email = $( 'txt_signup' );
	
	email.disabled = true;
	
    getXMLHttpRequestText( 
		( "send_email.php?function=signup&email=" + email.value ), 
		signup_callback );
}

function signup_callback( successful )  {
	if( successful )
		alert( 'Your email has been submitted successfully!' );
	else
		alert( 'Your email was not submitted successfully!' );

	$( 'txt_signup' ).value = "";
	$( 'txt_signup' ).disabled = false;
	$( 'txt_signup' ).focus();
}

function $( sId ){  return document.getElementById( sId );  }