You should bind the 'keyup' event and switch the status when pressing the SPACE key. Please try this code:
var sb = $('#sb');
sb.next().attr('tabindex',1).focus().bind('keyup',function(e){
	if (e.keyCode == 32){
		if (sb.switchbutton('options').checked){
			sb.switchbutton('uncheck');
		} else {
			sb.switchbutton('check');
		}
	}
});