There was an 8.something quake off of Sumatra at 8:09pm PST. This is in the same fault system where a magnitude 9.0 quake caused a massive tsunami that killed hundreds of thousands of people while displacing millions more.
Like the earlier quake, this quake was also situated along an undersea ridge. Depending on the movement of the earth's crust, this quake may also have unleashed a tsunami.
That this second quake happened was expected. Maybe not of such a great magnitude, but any given event of the magnitude of the original 9.0 quake causes enough motion within the earth's crust that it takes a long, long time for the fault system to settle again.
And the impact is not limited to that local fault system. With the original 9.0 event, the quake was felt in the south eastern United States.
Similar events have happened before. When the New Madrid Quake of 1811-1812 hit, it caused damage on the east coast of the United States and there were strong aftershocks for nearly a year.
So, unfortunately, it looks like the Indian Ocean and the surrounding countries are at high risk for further earthquake related damages. Hopefully, the December 26th, 2004 event will have put people on alert such that the various countries can react a little more quickly.
Someone asked how I knew about the earthquake before it had hit any major news services. The USGS has a site that provides maps dotted (squared?) with global earthquake information. There are also maps for specific regions. The global page has an RSS feed.
I happened to have been reading my morning feeds via NetNewsWires about 10 minutes after the initial tremor hit.