Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title></title>
<script>
window.onload=function(){
var tipo=['entrada','salida'];
for(var i=0;obj=document.forms[0].elements[i];i++){
if(obj.type=='radio' && obj.name=='rbTipo' && obj.checked==1){
alert(tipo[parseInt(obj.value)]);
break;
}
}
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<input name="rbTipo" type="radio" id="radio" value="0" checked="checked" />
<input type="radio" name="rbTipo" id="radio2" value="1" />
</form>
</body>
</html>