GSA misled me: exception was made for change.gov
Earlier this month I sent an email to the .gov registrar - part of the Government Services Administration - inquiring whether Obama's transition site change dot gov was within the federal rules for .gov domain names. They indicated it was.
However, the documents here - obtained through a FOIA request - indicate that "strings were pulled" and an exception was made at least regarding the domain name. It was originally considered too generic and it was also originally disallowed due to it being the same as a campaign's slogan.
That was until GSA Chief Information Officer Casey Coleman decided it would be "in the best interest of the Federal Government to register the subject domain name", after which the domain name was set up for the Obama transition.
As I know from registering non-.gov domain names, if your first choice is taken you can always think up dozens of other possible names. But, apparently the Obama team really wanted that domain name, and also apparently the permanent federal government really wants some change.