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');
}
}
});