Known unknowns are things we know we don't know. Part of the recent Rumsfeld Trilogy:

Known knowns - Things we know that we know.
    E.g. Saddam had weapons of mass destruction

Known unknowns - Things we know we don't know.
    E.g., What he did with them.

Unknown unknowns - Things we don't know we don't know.
    E.g., Well, this one is hard to offer an example of, since we don't know what they are.

The Unknown knowns are the ones he (Rumsfeld) left out. They would be things we don't know that we know and are almost as hard to show examples of as the Unknown unknowns, and for similar reasons.