можно так
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>demo</title>
<script type='text/javascript' src='http://code.jquery.com/jquery-git.js'></script>
<script>
$(function(){
var jCh = $(':checkbox[name="keyword"]'),
jText = $('#keyword');
jCh.change(function () {
var s = jCh.filter(':checked').map(function () {
return this.value;
}).get().join(' ');
jText.val(s);
});
});
</script>
</head>
<body>
<input type="checkbox" name="keyword" value="123" />123
<input type="checkbox" name="keyword" value="456" />456
<input type="checkbox" name="keyword" value="789" />789
<input name="keyword" type="text" size="12" title="" class="inputbox" id="keyword" />
</body>
</html>