Wouldn't anything they say be "hearsay" evidence, therefore inadmissable?
No. Hearsay is an out of court statement offered to prove the truth of the matter asserted. Let's take this example and pretend the proposed testimony of Bill Keller is "Judith Miller told me Valerie Plame was a CIA oficer." If that statement is offered to prove that Plame was indeed a CIA officer, it would be hearsay, because it would be offered to prove its truthfulness. If, however, that statement was offered to prove simply that Judith Miller told Bill Keller that Valerie Plame was a CIA officer, and not to prove that she actually was an officer, it would not be hearsay.