It turns out that John Ensign made his resignation effective the day before he was to testify under oath before the Ethics Committee:

Senator John Ensign’s resignation letter allows him to leave office just one day before he was to have to answer questions under oath about whether a $96,000 payment to the family of his former lover was illegal and designed to keep the affair from becoming public, according to people familiar with an investigation of Mr. Ensign’s activities.

That formal testimony, scheduled for May 4, was the final step as Senate investigators prepared for what were almost certain to be Senate ethics charges against Mr. Ensign, Republican of Nevada. Mr. Ensign’s resignation is effective May 3.

To paraphrase King Kong, it was cowardice and hypocrisy that killed the beast.