Jeff Moore talks about dependency injection in PHP and opines as to how he'd like to see "the current crop of frameworks" adopt the technique. Jeff, your wish is granted! (At least for Solar. ;-)
The calling code sends the dependency params to the target object at construction time. The target object then passes those params through Solar::dependency() to retrieve or create the dependency object.
The params may be an object (the dependency object itself), or a string (treated as a registry key for Solar_Registry::get()), or an array (which are treated as params to create a new dependency object internally at the target using Solar::factory()).
UPDATE (2009-03-28): Updating documentation links.