So which of the many well-documented prosecutorial missteps was most important in leading to the Justice Department's decision to drop the case against Ted Stevens?

The initial read, based on DOJ's statement, is that it was prosecutors' withholding of evidence from the defense.

Holder:

After careful review, I have concluded that certain information should have been provided to the defense for use at trial. In light of this conclusion, and in consideration of the totality of the circumstances of this particular case, I have determined that it is in the interest of justice to dismiss the indictment and not proceed with a new trial.

We'll have more on the details of this soon.