Entertainment Tonight.Barack Obama has been sworn in as the 44th President of the United States, with both U.S. government officials and Hollywood celebs looking on.During his speech, Obama acknowledged tough times, but struck a positive tone for the future. "Today I say to you that the challenges we face are real. They are serious and they are many. They will not be met easily or in a short span of time. But know this, America - they will be met.""What is required of us now is a new era of responsibility - a recognition, on the part of every American, that we have duties to ourselves, our nation, and the world, duties that we do not grudgingly accept but rather seize gladly, firm in the knowledge that there is nothing so satisfying to the spirit, so defining of our character, than giving our all to a difficult task," Obama said.Earlier in the day, outgoing President George W. Bush received the Obamas at the White House to give an official welcoming to their new home. Obama and his wife, Michelle, exchanged handshakes, smiles and kisses with Bush and his wife, Laura.After posing for a photograph, the foursome went inside to have coffee in the Blue Room with Vice President-elect Joe Biden, and his wife, Jill. In keeping with White House tradition, Bush left a note for Obama in his desk in the Oval Office, wishing him well as he assumes the presidency.The Bushes and Obamas then made their way to the U.S. Capitol via motorcade to the official swearing-in ceremony.Among those to be front and center at the inauguration were Oprah Winfrey, John Cusack, Dustin Hoffman, Steven Spielberg, Beyonce & Jay-Z and Muhammad Ali. The legendary "Queen of Soul," Aretha Franklin, belted out a moving rendition of 'My Country Tis of Thee" at the ceremony.