Instead of having to add the get_instance function to every class you want to use, I wanted to be able to extend an abstract class with just that functionality. This is the fruit of my efforts. You can read up more on it here:http://us1.php.net/lsb and here:

class-cur-abstract.php

<?php
abstract class Cur_Abstract {

    public static $instance;

    public static function get_instance() {
        // Instead of doing is_object( static::$instance ), we check to see if
        // static::$instance is an instance of static::$class. Because we're 
        // extending the class, static::$instance wants to be shared across any 
        // classes that extend Cur_Abstract, muddling up this check. If it were  
        // just is_object, it would pass, even though the instance is an 
        // instance of another class. 
        if ( ! ( static::$instance instanceof static ) ) {
		// This is where some of the magic is able to happen. Using Late Static 
		// Binding, we're able to refer to the extending class ( static ), rather than the
		// class being extended. In this case, static will be Cur_SomeClass when
		// it's being instantiated later on down the line.
		static::$instance = new static();
        }
        return static::$instance;
    } 
}

class-cur-someclass.php

<?php

include_once 'class-cur-abstract.php';

class Cur_SomeClass extends Cur_Abstract {

}

$someclass = Cur_SomeClass::get_instance();