Uhm, io avevo parlato di DIV, non di IFRAME.

codice:
<html>
	<head>
		<title>
			Nuova pagina 1
		</title>
	</head>
	<body>
		<form method="POST" action="...">
			<div style="display:block; visibility:hidden; position:absolute; width:300; height:50; left:250;t op:150; z-index:4;" id="attendi">
				Attendere...
			</div>
			


			
			</p>
			


			
			</p>
			


			<input type="button" value="Pulsante" onClick="document.getElementByID('attendi').style.visibility='visible';" name="B3">
			</p>
		</form>
	</body>
</html>