Here's the full text of Richard Perle's letter of resignation from the Defense Policy Board, written to Donald Rumsfeld. (Sorry, no link).
"Dear Mr. Secretary:
Thank you for finding time to see me on February 10. As I said then, the meeting of the Defense Policy Board would be my last, after 17 years of continuous membership under both Democratic and Republican administrations. It was a great privilege to serve on the DPB, but never more so than during your leadership. You have been open to ideas from all members of the group, challenging conventional wisdom and accepted doctrine and examining complex policy issues with great care, wisdom and courage. Since there is bound to be speculation about my departure from the DPB, I thought I would put what I said to you in writing.
Despite heated disclaimers, my membership on the defense policy board has led many people to see my articles, books and television appearances to associate my views with those of the administration or the Department of Defense. This results partly from a misunderstanding about the role and nature of the Defense Policy Board, exacerbated by the controversy surrounding policies I have advocated as a private citizen. A television viewer or newspaper reader, accustomed to zoning boards, school boards and appeal boards, is likely to think that the Defense Policy Board actually makes decisions, and that a member of it must be in a position to speak authoritatively about administration policy. The fact that the Defense Policy Board is not a decision making body but is simply a forum enabling the Secretary of Defense to hear a variety of opinions and observations (often opposed to one another) is simply not understood by the general public.
We are now approaching a long presidential election campaign, in the course of which issues on which I have strong views will be widely discussed and debated. I would not wish those views to be attributed to you or the President at any time, and especially not during a presidential campaign. This is particularly true now since I have just published a book that calls for far reaching reform of government departments responsible for combating terrorism. Many of the ideas in that book are controversial and I wish to be free to argue them without those views or my arguments getting caught up in the campaign. So it is with gratitude for the opportunity to serve the Department of Defense that I wish to resign from membership on the Defense Policy Board.
With admiration for your service to the nation and the cause of freedom, I remain, sincerely yours, Richard Perle"
To understand why Perle is a liability to Bush (and we don't include the American public's inability, in Perle's view, to comprehend what an advisory board does), the American Prospect's TAPPED blog assembles a formidable litany of links to Perle's various transgressions.