According to early reports (this doesn't mean they're accurate of course), it appears that a Nigerian national who studied in England (Times of London) flew from Nigeria to Amsterdam and then on to Detroit, where, on the landing descent, he tried to inject with a syringe a liquid into some dry material he had taped to his leg (ABC News).

According to passengers, this made popping noises and started a fire, at which point a Dutch man in his 20s or 30s -- described as "sturdy" -- subdued the suspect, and led him in a headlock to the front of the plane, where he was detained until he was arrested.

The resulting fire had to be put out with fire extinguishers, according to passengers.

The White House is calling it "an attempted act of terrorism."

What follows are different reports of the event...

From AP:

A Northwest Airlines passenger landing in Detroit on Friday tried to blow up the flight but the explosive device failed, two U.S. national security officials said.

The passenger, who was traveling on Northwest Airlines Flight 253 from Amsterdam, was not identified. He was being questioned Friday evening, according to one of the officials, both of whom spoke on condition of anonymity because the investigation was continuing.

The motive of the Christmas Day attack was not immediately clear.
...

Authorities initially believed the passenger had set off firecrackers that caused some minor injuries.

More info at Hot Air and at Malkin.

Update: AP sources: Al-Qaida link in failed plane attack:

U.S. officials say a Northwest Airlines passenger from Nigeria said he was acting on behalf of al-Qaida when he tried to blow up a flight Friday as it landed in Detroit.

Rep. Peter King, R-N.Y., identified the suspect as Abdul Mudallad, a Nigerian. King said the flight began in Nigeria and went through Amsterdam en route to Detroit.

One of the U.S. intelligence officials said the explosive device was a mix of powder and liquid. It failed when the passenger tried to detonate it.

Update II: Fox News reports that White House calls it "an attempted act of terror."

Update III: MSNBC's report here.

Update IV: Here's a Washington Post report.

Update V: Here's another report from the Detroit Free Press.

Update VI: Wall Street Journal: Midair Bomb Attempt Fails

Update VII: Here's a report from the New York Times.

Update VIII: The Times of London reports:

The suspect was "reported to be studying in Britain..."

Update IX: ABC News reports:

The suspect told authorities that he had explosive powder taped to his leg and used a syringe of chemicals to mix with the powder that was to cause explosion. This is of [concern] because it is a method of mixing that is consistent with terror techniques.