I looked it up:
http://www.boston.com/news/nation/articles/2004/09/08/bush_fell_short_on_duty_at_guard/On July 30, 1973, shortly before he moved from Houston to Cambridge, Bush signed a document that declared, ''It is my responsibility to locate and be assigned to another Reserve forces unit or mobilization augmentation position. If I fail to do so, I am subject to involuntary order to active duty for up to 24 months. . . " Under Guard regulations, Bush had 60 days to locate a new unit.
But Bush never signed up with a Boston-area unit. In 1999, Bush spokesman Dan Bartlett told the Washington Post that Bush finished his six-year commitment at a Boston area Air Force Reserve unit after he left Houston. Not so, Bartlett now concedes. ''I must have misspoke," Bartlett, who is now the White House communications director, said in a recent interview.
...
Lechliter said the records push him to conclude that Bush had little interest in fulfilling his obligation, and his superiors preferred to look the other way. Others agree. ''It appears that no one wanted to hold him accountable," said retired Major General Paul A. Weaver Jr., who retired in 2002 as the Pentagon's director of the Air National Guard.