Brandon Savage in his frustrations with the Zend Framework and the "self" keyword in PHP has written up a new post showing how you can use late static binding to work around it.

he problem is, when extended, My_Auth::getInstance() still returns an instance of Zend_Auth. The solution was to duplicate the static method in my My_Auth class, which worked properly. What did I get as a return value? Zend_Auth [...] Why didn't I get an instance of My_Auth instead of Zend_Auth? Well, that's because PHP determines the meaning of the self keyword at compile time, meaning that when you call a function that makes use of it later, you'll get whatever it's been defined to mean when it was compiled.

To remedy the situation he uses late static binding (in PHP 5.3+) by using the "static" keyword like you would use "self" to refer correctly to the current class, not the class it sees at runtime.