jQueryでチェックボックをONとかOFFするとかなんとか

これでどう？

<script type="text/javascript" src="http://www.google.com/jsapi"></script>
<script type="text/javascript">
google.load("jquery");
google.setOnLoadCallback(function(){
	$(document).ready(function(){
		$('#selectAll').click(function(){
			if($('#selectAll').val() == 'select all'){
				$('#selectAll').val('clear all');
				$('[type="checkbox"]').each(function(){
					$(this).check();
				});
			}else{
				$('#selectAll').val('select all');
				$('[type="checkbox"]').each(function(){
					$(this).uncheck();
				});
			}
		});
	});
	jQuery.fn.extend({
		check: function() {
			return this.each(function() { this.checked = true; });
		},
		uncheck: function() {
			return this.each(function() { this.checked = false; });
		}
	});

});
</script>

トグルするならこれとか

<script type="text/javascript" src="http://www.google.com/jsapi"></script>
<script type="text/javascript">
google.load("jquery");
google.setOnLoadCallback(function(){
	$.fn.extend({
		toggle: function() {
			return this.each(function() { this.checked = !this.checked; });
		}
	});
});
</script>