Quicksilver supports Growl via a plug-in.
In Quicksilver's Application menu, select the "Get More Plug-ins..." item.

You can install from the web using the plus icon — or download (and double-click) the plug-in using the arrow icon.

Quicksilver will use Growl to notify you that the plug-in has been installed.

Other notifications that the Quicksilver plug-in sends are iTunes track changes and when you use Quicksilver to switch network locations.