Adium contacts aren't really valid Spotlight results, nor is Spotlight really intended to be a flexible launcher ala gnome-do, Alfred, Quicksilver, Launchbar, etc.
i'm not aware of any launchers with this capability at the moment. Alfred, Quicksilver, and QSB are the only ones i'm intimately familiar with though. The Alfred developer is adding features fairly quickly, you could ping him with the query and see what he says. I usually just use Alfred to bring Adium to the front then use cmd-n to bring up the "new chat" dialog and start typing the name until autocomplete finishes it for me.
Update: From the Alfred Developer:
@peelman this is definitely on the list, will link into the new contacts integration too :)