UPDATE 2/19/2009 9:45 AM: Well, it seems that my hunch was at least partially right. Reuters (and Paula Narvaez, special envoy to the Chilean president) is calling the eruption as result of "what appeared to be a partial collapse of its cone." So, we might have seen the oversteepening of the dome growing in the Chaiten caldera that lead to a collapse, producing (likely) a pyroclastic flow and either an accompanying plinian eruption as the pressure was released or an ash column associated with the pyroclastic flow itself. Now, I might not take Reuters word for it, but it makes sense considering the suddenness of the event.
10:00 AM: More details for Reuters, including the ominous "Our security team have observed an increase in the size of a column of ash and smoke, with a deformation to one side" from the Deputy Interior Minister. My guess is he is referring to deformation on the ash column, not the volcano.
4:00 PM: From a Newsday article, just some more details on the ash dispersal: "On Thursday, increased seismic activity was reported and ash fell 100 miles (160 kilometers) away in Futaleufu."
Sure enough, the Associated Press also reporting that an explosion has occurred in the main dome of Chaiten that has been built since last May and that material is moving downslope from the explosion. There is no clear word what type of volcanigenic material this might be - pyroclastic flow, lahars, avalanche - but there is a major fear that it could block the river and cause flooding in the town of Chaiten. The few citizens left in the area are being evacuated. The Patagonia Times adds that the eruption started around 11 A.M. (local time) and ash has spread across much of the region.
From what I can gather, this seems like it might have been either a dome collapse that was quickly followed by an explosion (possibly caused by the release of pressure from the collapse) or some "burp" of gas-rich magma erupting. The reports of "a massive column of ash" are interesting as this could suggest a true new flux of magma, but it is hard to tell at this point how widespread is the ash and how tall the column might be. At this point, it is all conjecture, but it sounds like Chaiten might be making a comeback. The questions woulds be how big and for how long.
I'll update as we find out more ...