An interesting perspective:

If the 111th FIS and Lt. Bush did not go to Vietnam, blame President Johnson and Secretary of Defense Robert S. McNamara, not lowly Lt. Bush.

But isn't the concern over whether or not Bush personally recieved special treatment, not over his whole unit's duties?

If you check the 111th FIS records of 1970-72 and any other ANG squadron, you will find other pilots excused for career obligations and conflicts. The Bush excusal in 1972 was further facilitated by a change in the unit's mission, from an operational fighter squadron to a training squadron with a new airplane, the F-101, which required that more pilots be available for full-time instructor duty rather than part-time traditional reservists with outside employment

(from The Washington Times: Letters to the Editor - Editorials/OP-ED). If that's true, then Bush did not recieve any special treament and fufilled his duties.