From the White House blog:
In September 2009, the President announced that—for the first time in history—White House visitor records would be made available to the public on an ongoing basis. Today, the White House releases visitor records that were generated in October 2011. Today’s release also includes visitor records generated prior to September 16, 2009 that were requested by members of the public in December 2011 pursuant to the White House voluntary disclosure policy. This release marks a new milestone, bringing the total number of records made public by this White House to more than two million—all of which can be viewed in our Disclosures section.
via More Than Two Million Records Released | The White House.