There are two different, and distinct extensions for PHP, the older memcache, and a newer memcached extension. Even though they perform the same primary function - connect to a memcached instance somewhere - they are not interchangeable! They use different PHP function calls. Phalcon currently uses the more prevalent memcache extension and it's functions. If you have only installed the memcached extension and libmemcached, the Memcache classes will not function!
We all need to be VERY careful with what we call classes and the like. For instance, there is a class in the incubator that is called Phalcon/Cache/ Backend/Memcached.php, but then uses
use Memcache as NativeMemcache;
$this->_memcache = new NativeMemcache;
This is NOT using the memcached extension with libmemcached as one would assume by the name.
If you do a Google search, you will see that memcached is the newer, faster extension that implements more of the libmemcached protocols. It WOULD be nice if the memcached extension was supported in the C code. You can write your own of course by extending the appropriate classes, but having it in user-land defeats the purpose of a compiled framework yes? :-)