SO jQuery Fiddle
Test Fiddle mainly for SO Questions jQuery 1.4.3, UI 1.8.5
by Nick Craver
HTML
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/themes/ui-lightness/jquery-ui.css">
<input name="mytest" type="checkbox" value="1">
<input name="mytest" type="checkbox" value="2">
<input name="mytest" type="checkbox" value="3">
<input name="mytest" type="checkbox" value="4">
JavaScript
$("input[name='mytest']").val([1, 2, 3]);