Mullah Mohammed Omar was and is the leader of the Afghani Taliban from before September 11, 2001. He was an active ally of Osama bin Laden and al Qaeda and under his wing the 9/11 atrocities were planned. The United States would be making a dear mistake believing that this war criminal is a legitimate target for negotiation.
Earlier this month the German weekly Der Speigel reported that Germany had helped U.S. officials contact Mullah Omar's personal secretary, Tayyab Aga. He was the last public voice of the Taliban before fighters fled southern Kandahar province in December 2001, shortly after U.S.-led invasion. While Germany has been involved, opening of contact with Aga was an American initiative, a western diplomat in the region told The AP.
If anything, Omar may consider some sort of "conditional" talks in which the US withdraws some (or most) of its forces for a Taliban cease-fire. Then when the US, ISAF, and Afghan government are weak, launch a Tet-style offensive to finish off international involvement.
Furthermore, even if Omar was a legitimate partner (which he certainly is and will not be) he does not control every cell of the Taliban. The decentralized nature of the Islamist group does not allow one leader to control every aspect of the organization. Even if it did, independent cells or individuals would not adhere to any peace plan and would attack ISAF and Afghan forces.
This is a terrible plan, attempted as a fig leaf for "withdrawal." The White House is attempting a peace with honor plan by "negotiating" with the most dishonorable (and bloody) man in Afghanistan.