I don’t see what’s so difficult about this. Executive privilege is a constitutional issue having to do with separation of powers. On top of that, there is the matter of lawyer-client privilege.
The headline is misleading. Bush is not ordering Miers not to testify, he is confirming that she cannot be subpoenaed to testify because congress lacks the jurisdiction to do it.
If congress doesn’t like it, they can go shovel sand.
I doubt if any of them would know HOW to shovel sand. Their poll numbers are so low I wonder how they can even lift their heads.