I was having trouble making method calls with the result callbacks in getDelayed, so I emailed the developer.
If you want to use a non-static method as a callback, use the following format: array($obj, 'method'); for example:
<?php
class foo {
    private $M = false;
    
    public function __construct() {
        $this->M = new Memcached();
        $this->M->addServer('localhost', 11211);        
        $this->M->set('a', 'test');
    }
    public function test() {
        $this->M->getDelayed(array('a'), false, array($this, 'fun'));
    }
    
    public function fun() {
        echo "Great Success!";
    }
}
$f = new foo();
$f->test();
?>
or, alternatively:
<?php
class foo {
    public $M = false;
    
    public function __construct() {
        $this->M = new Memcached();
        $this->M->addServer('localhost', 11211);        
        $this->M->set('a', 'test');
    }
    
    public function fun() {
        echo "Great Success!";
    }
}
$f = new foo();
$f->M->getDelayed(array('a'), false, array($f, 'fun'));
?>
Works great, thanks Andrei :)