Links at the top of the Drudge Report point to two preliminary stories: WNBC and Breitbart. There's nothing on the local news in Philadelphia save a feed on CBS 3 from a chopper showing billowing smoke, but this is done picture-in-picture while the regular program continues.

Details are few and sketchy (from WNBC):

A three-story building on Manhattan's East Side went up in flames and collapsed Monday after what witnesses said was a thunderous explosion that rocked the neighborhood.

The cause was not immediately known, and at least two people were under evaluation at a hospital.

...

TV host Larry King, who had been in his hotel room nearby, described the explosion to CNN as sounding like a bomb and feeling like an earthquake.

The reporter had the sense to include a pertinent detail in light of recent threats: the subway was unaffected.

UPDATE: From SFGate: "The cause was not immediately known, though White House press secretary Tony Snow said there was no indication of terrorism."

UPDATE 2: Salt Lake City's CBS affiliate says: "Con Edision was investigating a smell of natural gass in the area earlier in the morning, according to WCBS-TV reports."

UPDATE 3: The latest Drudge link suggests a doctor's attempted suicide.