Not exactly volcanic, but...
Not many details yet, but the USGS Earthquake list shows a M7.2 earthquake ~25-30 km from Christchurch, New Zealand. I'll post more details as I find them, but here is a Google Map of the epicenter - UPDATE: the refined location puts it further from Christchurch, to the northwest. The earthquake appears to be relatively shallow - 16-30 km deep, but that is a preliminary estimate. I don't know much about the tectonics/structure around Christchurch, but you can see that Christchurch is to the south of the termination of the subduction zone between the Australian and Pacific plates. The historic seismicity for the area implies this isn't a likely location for a big earthquake. There is no active volcanism on the South Island of New Zealand (that I know of ... anybody know better?)
UPDATE 9/4: If you are looking for the best and most indepth coverage for the science aspects of the earthquake, head over the Highly Allochthonous. It is remarkable that no one died in this earthquake considering its magnitude and location. You can see the video of the damage on the NZ Herald site - lots of examples of surface displacement - and other updates as well.
An amazing gallery of images from around Christchurch - Some of the images of the damage to roads is stunning.
And if you're looking for images, here is one of the first.
Here is an abstract for an article on the geology around Christchurch (Thanks to Stephen)
CNN International has a running article on the effects of the quake.
Some more reports of damage in Christchurch.
And here are some of the first details from Radio New Zealand: It was also felt in other parts of the South Island and in Wellington, where it was felt in a rolling motion which lasted for about 40 seconds.
Radio New Zealand listeners report particularly strong effects in Christchurch, with one resident saying the turned his house upside down, with crockery breaking.
Another said the power had gone out.