Код работает нормально
<!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=utf-8" />
<title>Документ без названия</title>
</head>
<script type="text/javascript">
function myfoo(b){
if(b.className=='readable'){
b.className='editable';
b.readOnly=false;
}else{
b.className='readable';
b.readOnly=true;
}
if (b.readOnly){
alert('Read Only!');
}
}
</script>
<style>
.readable {
border-style:outset;
}
.editable {
border-style:inset;
}
</style>
</head>
<body>
<form action="" method="">
<input id="id_1" name="ai1" ondblclick='myfoo(this);'/>
</form>
</body>
</html>