danik.js,
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<style>
.hidden { display: none; }
</style>
<script src="http://code.jquery.com/jquery-1.11.0.min.js"></script>
</head>
<body>
<form id="form">
<input type="checkbox" id="cb1"/> <label for="cb1">Checkbox 1</label>
<input type="checkbox" id="cb2"/> <label for="cb2">Checkbox 2</label>
<input type="checkbox" id="cb3"/> <label for="cb3">Checkbox 3</label>
<input type="checkbox" id="cb4"/> <label for="cb4">Checkbox 4</label>
<button id="delete" class="hidden">Удалить отмеченные</button>
</form>
<script>
$(document).ready(function(){
$('#form input:checkbox').change(function () {
$('#delete').toggle($('#form input:checkbox:checked').length>0);
});
});
</script>
</body>
</html>
так?