Possibilities:
Most likely: Faulty field diode in alternator.
Or: Wiring fault between ignition switch and alternator usually caused by poor fitting of accessories into the harness.
Either fault will usually allow the alternator to charge the battery normally. The problem is that in the event of the alternator drive belt snapping you wouldn't take any notice as the warning light is already on.
So: either put some tape over the warning lamp to stop it blinding you at night and check the alternator drive belt regularly or try to locate the cause of the problem.
Dann
October 2007