Sorry, war nen kleiner Fehler beim <select>, nämlich onchange und selected fehlte :-)
<body>
<form method=POST action=send.php name=call>
<input type=hidden name=id value=1>
<script type=text/javascript>
function man_sel() {
if(document.call.next_call.selectedIndex==0) {
inp = document.createElement("input");
inp.style.width = "120px";
inp.setAttribute("type","text",true);
inp.setAttribute("name","next_call",true);
inp.setAttribute("value","<? echo date('d.m.Y'); ?>",true);
pos = document.getElementById('next_call');
pos.replaceChild(inp,document.call.next_call);
}
}
</script>
<span id=next_call>
<select name=next_call onchange=man_sel()>
<option>Anderes Datum eingeben</option>
<option selected>01.01.1970</option>
[...]
</select>
</span>
<input type=submit value=Senden>
</form>
</body>