This possibility that Bush could be removed from office on the grounds of being mentally incompetent to perform his duties as President was discussed in July 2005 by Jeffrey Steinberg in his article The Plame Affair: Rove and Cheney Are Guilty As Charged:
Twenty-fifth Amendment
http://en.wikipedia.org/wiki/Twenty-fifth_Amendment_to_the_United_States_Constitution
Section four: Involuntary withdrawal
It is also possible for the Vice President, together with a majority of the heads of the executive departments (that is to say, members of Cabinet) or of such other body as Congress by law provides, to declare the President disabled. The provisions of section four have never been invoked. An "involuntary" withdrawal may be declared for a variety of reasons. The most likely scenario, and the primary purpose of section four, is incapacitation. This results in both the inability of a president to discharge the duties of his office, and his inability to provide a written declaration to that effect. It is possible, however, for such a declaration to be made even if the president is fully able and conscious, should the majority of his officers find grounds other than medical disability, such as insanity, emotional instability, or conflict of interests, affecting national security. In any event, the president would continue to discharge the powers and duties of his office until Congress had reviewed the case and voted to strip the president of his powers, thereby preventing the president's officers from exploiting section four to stage an easy coup. The President may resume his duties by a written declaration sent to the President pro tempore and the Speaker. If the Vice President and Cabinet, however, are still unsatisfied with the President's condition, they may within four days of the President's declaration submit another declaration that the President is incapacitated. Congress must decide within 21 days the issue; a two-thirds vote in each House is required to permit the Vice President to assume the Acting Presidency. For the remainder of his term, the elected president remains the President (and the acting president remains the Vice President) albeit stripped of his powers and duties, and he may compell Congress to review his competence at any point in order to resume his duties.