It's Joe Biden

Confirmed via CNN at 12:45am EST.

Earlier tonight:

The United States Secret Service has dispatched a protective detail to assume the immediate protection of Sen. Joseph Biden, D-Del., a source tells ABC News, indicating in all likelihood that Biden has been officially notified that Sen. Barack Obama, D-Illinois, has selected him to be his running mate.

Sources also tell ABC News that two others said to be finalists for the position of Obama's running mate -- Sen. Evan Bayh, D-Ind., and Virginia Gov Tim Kaine -- have been told they have not been selected for the vice presidential slot.Kaine flew to Denver earlier today, is in Denver tonight and has no plans to leave in the next 24 hours. Bayh is at his Washington, DC, home with his family.

Biden was last seen minutes ago at his Wilmington estate where his extended brood is gathering.

UPDATE: Via New York Times:

Senator Barack Obama has chosen Senator Joseph R. Biden Jr. of Delaware to be his running-mate, turning to a leading authority on foreign policy and a longtime Washington hand to fill out the Democratic ticket, people told of the decision said.

Mr. Obama’s selection ended a two-month search that was conducted almost entirely in secret. It reflected a critical strategic choice by Mr. Obama: To go with a running-mate who could reassure voters about gaps in his resume, rather than to pick someone who could deliver a state or reinforce Mr. Obama’s message of change.