It was pointed out to Craig that his airport arrest was likely unconstitutional, because Article I Section 6 Clause 1 says:
"They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place."
Since Craig was traveling back to Idaho after the close of a session of the Senate, it was thought that he was immune to arrest.
-PJ
Larry Craig announced his resignation in September 2007 but did not leave office until January 3, 2009 the very same day his term ended.
Leaving your seat when your term is over is not considered a resignation.