From the New York Times:
Libby and Cheney Will Not Testify, Says the Defense
Libby will not call Dick Cheney to the witness stand, so the vice president won't have to go under oath and answer questions about his involvement in exposing a CIA officer. The only reason I can think of that Libby would fail to call Cheney to testify for him is that Libby knows that if he's convicted, Bush will pardon him before leaving office.