This excerpt taken from the VZ DEF 14A filed Mar 23, 2009.
Service on Other Boards
A Director who serves as an executive officer of a public company should not serve on more than three public company boards, including that of his or her own company, and other Directors should not serve on more than six public company boards. In order not to disrupt existing affiliations with other boards, a Director may maintain current positions, even if the number of those positions exceed these limits, unless the Board determines that permitting the position to continue will impair the Directors ability to serve on the Verizon Board. Directors are expected to advise the Corporate Governance and Policy Committee of any changes in their membership on other boards.