There have been cases of congressmen being sued. Sen. Proxmire, most famously.
An interesting wrinkle in the Murtha case is that he made all those false allegations from his campaign headquarters in Johnstown, Pennsylvania. That's part of the argument that Murtha was acting outside his role as a congressman.
I would think that it'd be clear enough that no elected representative has the right to falsely accuse citizens of being cold-blooded murderers. But President Bush's DOJ apparently thinks otherwise.
Mr. Bush's DOJ is constitutionally wrong:
Article I Section. 6.
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States.
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.
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.
Murtha's statements were made, not only on the Floor of the House, but in a public venue both before and after his Floor speeches. Also notice that "Treason, Felony and Breach of the Peace" ARE causes for arrest on the Floor.