Responding with my own answer to add-on to Adrian's, and the question you asked in response.
If your service has dependencies of it's own, you just use a factory instead of going the invokable route.
Say your service needs a cache adapter and database adapter. Also imagine that it can optionally be configured with some other service (FooService, below):
<?php
public function getServiceConfig()
{
return array(
'factories' => array(
'my_service' => function($sm){
$cache = $sm->get('Cache');
$dbAdapter = $sm->get('DefaultDbAdapter');
$fooService = $sm->get('FooService');
// instantiate your service with required dependencies
$mySvc = new \My\Shiny\Service($cache, $dbAdapter);
// inject an optional dependency
$mySvc->setFooService($fooService);
// return your shiny new service
return $mySvc;
}
)
);
}
Side Note: It's generally bad design to inject the ServiceManager all over the place. You're better off managing your dependencies more explicitly, like above.
This stuff is covered quite well in the Quick Start, if you haven't already read that.