NATO has taken over command of the no-fly zone. However, US aircraft are still very much involved according to this report from the Air Force Association:
While NATO is now in charge of air operations over Libya, US military aircraft continue to play a critical role in the mission of protecting Libyan civilians and enforcing the no-fly zone, said alliance officials Tuesday. Alone on Monday, a US fighter aircraft destroyed two of Libyan dictator Muammar Qaddafi's surface-to-air missile sites near the Libyan capital of Tripoli, they said. This is in addition to the daily aerial refueling, ISR, and command and control sorties that US assets fly. Since NATO took over command responsibility for the Libyan mission, now called Operation Unified Protector, on April 1, US military platforms have flown more than 800 of the nearly 2,900 total air sorties. Of those 800, more than 150 of them have been to suppress Qaddafi's air defenses. In eight of those, the US aircraft dropped ordnance, according to the officials.
The more cynical amongst you might conclude NATO's role is merely to replace the US as NATO lacks the depth to truly replace US forces. Those same cynics might also point out the most vocal opponent to Qaddafi is France which is not a member of NATO. At over $680 million and counting, I'm not sure what the United States has gotten out of Odyssey Dawn.