Use .triggerHandler() instead of .trigger():
$('input[name=check1]').attr('checked', true).triggerHandler('click');
Also, use .prop() instead of .attr():
$('input[name=check1]').prop('checked', true).triggerHandler('click');
(if you're using jQuery 1.6 or newer.) edit — Also, as I commented on another answer, you have to watch out for jQuery's weird behavior when programmatically triggering events. Here is an illustrative jsfiddle. When a real "click" happens on an element, the "click" handler for the element will see the updated value of the "checked" flag. That is, if you click on an unchecked checkbox, the click handler will see the "checked" flag set to true. However, if you trigger "click" on an unchecked checkbox via jQuery, the "click" handler will see the "checked" flag set to false! That's a really bad thing, in my opinion, but it's always done that.
edit again — oh, also, I forgot another important (and irritating) jQuery weirdness: for reasons unknown, the .triggerHandler() API will only invoke handlers on the first matched element. If you try to trigger all the checkbox "click" handlers, in other words:
$('input:checkbox').triggerHandler('click');
then only the first checkbox on the page will be triggered. What I generally do in order to deal with the insanity is bind the handler with my own fake event name as well as "click":
$('input:checkbox').bind('click my-click', function() ... ) // or ".on()" with 1.7
That way I can trigger "my-click" and get the best of both worlds: the library triggers the handler on all the matched elements, but it won't toggle the actual state of the elements.
