The government's decision to release the size of the nuclear stockpile is an important milestone in the history of nonproliferation. It is more than a symbolic gesture because the U.S.'s refusal to acknowledge the number of weapons it has has been regularly given as an excuse for why other countries aren't fulfilling their international obligations. (Update: 10-05-03_Fact Sheet_US Nuclear Transparency _FINAL.PDF)
The reality is that the U.S. stockpile now is smaller than its been since the Eisenhoweradministration. That fact alone is an important demonstration of the United States'
commitment over a long period of time to reducing its stockpile, something that is required by the Nuclear Proliferation Treaty's Article Six.
Problem is, the U.S. has claimed to be reducing its stockpile since the end of the Cold War, but it hasn't been able to back up its claims because the intelligence community refused to declassify details about the stockpile. For example, the U.S. can say that it's dismantling weapons at an increasing rate, but it can't say what the rate is, or what the final number would be
During the administration's internal debate about the Nuclear Posture Review, there was a near consensus to disclose it then and there. At the last minute, the intelligence community asked for more time to study the ramifications of its release, and the National Security Staff decided not to object. The fault lines remain, but the N.S.S. believes that disclosure is warranted. (This is one reason why the administration leaked news of the decision early...they wanted to ratify the decision in the press before the IC could object.)
As The New York Times reports today, the IC was specifically worried that by acknowledging the size of stockpile, terrorists and nuke-seeking countries could extrapolate critical data. As the Times helpfully points out, the data is ubiquitous. In case that's not clear enough, the Times actually provides the data.