As noted in this thread, if Rove or Miers should fail to comply with a Congressional subpoena, they can be prosecuted for contempt of Congress. Then the U.S. Attorney for the District of Columbia would ask a grand jury for an indictment – except that that official reports to AG Gonzalez, who might give orders not to proceed. A sticky conflict of interest.
One way to cut the knot would be the appointment of an Independent Counsel – but the authorizing statute expired in 1999. Maybe now would be a good time for Congress to revive the office?