It depends on the situation. Take the 1994 Philippine Air bombing by Ramzi Yousef. That went unclaimed because they were trying a new method (liquid bomb) that they intended to use on a grand scale (Operation Bojinka).
It could be something like that. It's been a long time since they've been able to successfully bring down a commercial plane in the western hemisphere (within striking distance of the US). If they've developed a new method that works, I could understand why they'd keep quiet about it for the time being.
That's just one possible scenario. There are others.
Oh Geez..that is a scary thought. Makes sense though.
From the Wikipedia article on the Philippine Air 443 bombing, which killed the person sitting in the seat where it was planted:
The seat where the bomb exploded (seat 26K) would normally be above the center wing fuel tank on an older Boeing 747. However, on the SAS Version model of the 747, used in this flight, seat 26K was two seats forward of the tank. If the bomb exploded in a horizontal manner it would have punctured the aircraft skin, causing explosive decompression. Instead, the explosion occurred in a vertical manner running front-to-back on the plane; [Japanese businessman Haruki] Ikegami's body took the brunt of the force, people immediately in front and behind were injured, and the sudden vertical expansion of the cabin severed steel cables controlling the 747's rudder and elevator located in the aircraft ceiling. The bomb also severed the copilot's control cable for the right aileron....
United States prosecutors said the device was a "Mark II" "microbomb" constructed using Casio digital watches as described in Phase I of The Bojinka Plot of which this was a test. On Flight 434, Yousef used one tenth of the explosive power he planned to use on eleven U.S. airliners in January 1995. The bomb was, or at least all of its components were, designed to slip through airport security checks undetected. The explosive used was liquid nitroglycerin, which was disguised as a bottle of contact lens fluid. Other ingredients included glycerin, nitrate, sulfuric acid, and minute concentrations of nitrobenzene, silver azide, and liquid acetone. The wires he used were hidden in the heel of his shoe. At that time, metal detectors used in airports did not go down far enough to detect anything there.
AF 447 took unusually long to die for a bombing. Usually, a bomb causes instantaneous breakup, leaving no time for distress signals, automated or otherwise. However, a weak bomb such as the one described above might be consistent with what's known so far.