Bush is using the executive privilege dodge to play out the string, dragging every Congressional request for testimony or information into court where it will be delayed until he leaves office. The concept of "executive privilege" is a phantom. It exists nowhere in the Constitution and is never mentioned in the Federalist Papers that explained the Constitution to the public 200+ years ago. It was the twisted construct of the Nixon Supreme Court and, as such, is an ill-defined joke on the Congress.
Tying Congressional subpoenas to an impeachment investigation turns the executive privilege dodge into obstruction of justice. Congress has an absolute right to investigate whether the President has committed high crimes or misdemeanors. No court can block such Congressional action. If Bush refuses to comply with such subpoenas that fact alone is a felony and sufficient cause for removing him from office.
It is called playing hardball.