From my reading of the indictment, there does not appear to be a dispute between Libby and the prosecutor as to the fact of whether Cheney informed Libby of Plame's CIA affiliation before Libby spoke with reporters.
Libby admitted in his statements that he did learn this first from Cheney. What Libby goes on to say is that he sort of forgot that he had been told it by Cheney and that it seemed to him he was learning about Plame as if it were new when Russert told him.
So it probably is NOT necessary, strictly speaking, that Cheney testify. Both sides could stipulate as to what and when Cheney told Libby.
However, Libby may want to call Cheney to testify to provide more information about the circumstances of the conversation.
Recall that there are really two aspects of the alleged false statements by Libby.
First, there is the he said/she said between Libby and each of the three reporters. The fact that Russert and Cooper can be shown to have a background as (or are married to) partisan Democrats, and the inherent difficulty in very busy executives recalling with exactitude conversations that occurred months earlier, will be likely to raise a reasonable doubt in the minds of the jurors, so long as the jury is not essentially a jury of Democrats (the venue will be important to Libby - he should try to get it out of D.C.). And I think Miller will just come across as a dingbat.
I actually think the more serious of the two aspects of the false statements alleged to have been made by Libby is the second one: that Libby claimed when he discussed the fact of Plame with reporters he heard it "as if it were new" and forgot having been told it by Cheney. The problem with this statement is that it may seem unbelievable in light of the fact that, according to Fitzgerald, he discussed Plame with about six other people in the White House, State and CIA prior to speaking with the reporters. If only one person, such as the VP, had mentioned it to Libby in passing in the midst of a conversation concerning mainly other topics, then it would be more believable that Libby might have forgotten.
So it may be in Libby's interest to call the VP and the various other people who told the FBI or the GJ that they had discussed Plame with Libby, in order for Libby to put these conversations in context and demonstrate to the jury that the information about Plame was just one small bit of data that was floating around in his mind and not a very important and a quite forgettable fact.
A side benefit to Libby of calling Cheney as a witness is to put into play issues of executive privilege that may take months or years to fully litigate. It is in Libby's interest to slow things down, as I believe the further back in time the events become the less likely it is that the jury will want to jail anyone over this and if it drags on until January 2009 perhaps the president will pardon Libby before he would have to serve jail time.
I enjoyed reading and thinking through your very astute response. Thank you.