I have found myself doing this in my code to 'cache' the work done when instantiating my Zend_Db_Table models:

if (Zend_Registry::isRegistered('x_table')) {
    $x_table = Zend_Registry::get('x_table');
} else {
    $x_table = new Default_Model_DbTable_X;
    Zend_Registry::set('x_table', $x_table);
}

It bothered me that this method isn't very DRY and it dawned on me today that a singleton pattern would probably be a better way to do this. Problem is, I've never written a singleton class. When I did some web searches, I found some offhand comments about Zend_Db_Table singletons, but no real examples.

I already have meta-data caching configured.

1. How do I make my Zend_Db_Table models singletons?
2. Are there pitfalls or downsides?