<html>
<head>
<script language="JavaScript">

var dAtt
dAtt=1
var fx

function init() {
fx=setInterval("changeF()",3000)
}

function changeF() {
id (dAtt==1) {
frase1.style.visible="hidden"
frase2.style.visible="visible"
dAtt=2
} else {
frase2.style.visible="hidden"
frase1.style.visible="visible"
dAtt=1
}
}

</script>
</head>
<body onload="init()">
<div id=frase1 style="position:absolute; top:50px; left:100px; visibility:visible">
CIAO
</div>
<div id=frase2 style="position:absolute; top:50px; left:100px; visibility:hidden">
Come va?
</div>
</body>
</html>

credo ke funzioni... prova^^