It's not explicit, just like all sorts of other ranking details and polcies are secret, to keep manipulators in the dark as long as possible.
From http://googlesystem.blogspot.com/2008/11/google-searchwiki-l...:
"At this time we aren't using SearchWiki to influence ranking but it is easy to see how that could happen in the future," says Marissa Mayer.
I suppose they are going to wait until they get a meaningful amount of user data before they start using it.
It's not explicit, just like all sorts of other ranking details and polcies are secret, to keep manipulators in the dark as long as possible.