Doesn’t look like it’s related to his wiretapping reporting:

Mr. Risen’s lawyer, David N. Kelley, who was the United States attorney in Manhattan early in the Bush administration, said in an interview that the subpoena sought the source of information for a specific chapter of the book “State of War.” The chapter asserted that the C.I.A. had unsuccessfully tried, beginning in the Clinton administration, to infiltrate Iran’s nuclear program. None of the material in that chapter appeared in The New York Times.

I blogged about this possibility back in 2006:

If New York Times reporter James Risen is subpoenaed like Judith Miller and Matthew Cooper were during the investigation into who leaked the identity of Joseph Wilson’s wife, you’re going to hear the same arguments from the NYT’s lawyers that you heard then — reporters should have a right to protect their sources because their sources are brave, selfless whistleblowers who are doing a public service. As the Russ Tice case indicates, the truth is often more complicated.