If you generate checkboxes and labels dynamically, or just don't feel like assigning ids to all of your checkboxes and corresponding for attributes to all of your labels, here's a quick jQuery snippet to take care of this:
$('input[type="checkbox"] + label').click(function() {
$(this).prev().attr('checked', !$(this).prev().attr('checked'));
});
This has the added advantage of minimizing your HTML markup.