The reddit json contains two objects: the post, and the comments. The comments are located at data[1] This should work:
$.getJSON("http://www.reddit.com/r/" + sub + "/comments/" + id + ".json?", function (data){
$.each(data[1].data.children, function (i, item) {
var comment = item.data.body
var author = item.data.author
var postcomment = '<p>[Author]' + author + '<br>' + comment + '</p>'
results.append(postcomment)
});
});