In the Quicksilver preferences, uncheck the 'show icon in dock' option (requires restart). After relaunching it, drag the icon from your Applications folder into the Dock. Quicksilver's icon will remain there, albeit without the black triangle indicating it has been launched. Notice that Quicksilver does not appear in the Application switcher (Command-Tab), however you can still perform drag 'n' drop operations.
This hint presumably works for any application that lets you run it without a Dock icon.

