From Dr Jeff Master's Wunderblog:
The Eyjafjallajökull volcano on Iceland erupted Wednesday, sending a towering cloud of volcanic ash 6 - 11 km (20,000 - 36,000') high in the air from its 1666 meter (5500') high peak. The ash cloud has caused a dramatic interruption of air traffic over much of northern Europe today, and this disruption will spread southwards and eastwards over the next day as the the ash cloud gradually spreads and disperses. (click here for the remainder of Dr Jeff Master's blog entry on this eruption as well as a followup)
It's still too early to speculate how this eruption may impact the global climate over the next few months and years. This initial eruption was too small to have a significant climate influence, especially for a high latitude eruption which tends to get washed out of the atmosphere more quickly than volcanic eruptions over tropical latitudes. However, if this volcanic eruption is sustained for weeks and months (like the Laki Iceland eruption of 1783-4), then it could have significant impacts on the global climate, especially over western Europe. Click here for more information on climatic impacts of volcanic eruptions.
More information available from the UK Met Office and ScienceBlogs . Be sure to check out the eyjafjallajokull webcam for live images of the eruption.
Update Apr 17: Some incredible new photos of the eruption, including this one showing lightning discharge from the ash cloud. Absolutely stunning. More amazing photos available at Stromboli online as well as from the UK Telegraph and Boston Globe.