And here is a video that gives you a sense of the size of the ash cloud:
Here is an article in the UK Independent that explains that despite being bigger than last year's Eyjafjallajökull eruption, the Grímsvötn eruption is not expected to cause air travel chaos.
I found this bit on Wikipedia to be interesting:
On 21 May 2011 at 19:25 UTC, an eruption began, with 12 km (7.5 mi) high plumes accompanied by multiple earthquakes. The ash cloud from the eruption rose to 20km/12 miles, and is so far 10 times larger than the 2004 eruption, and the strongest in Grímsvötn for 100 years.
During 22 May the ash plume fell to around 10 km altitude, rising occasionally to 15 km.
As of writing, the eruption has been releasing about 2000 tons of ash per second, making it 120 million tons in the first 48 hours. This makes the 2011 eruption of Grimsvotn a confortable VEI4 in the scale of volcanic explosive index (VEI), releasing more ash in the first 48 hours than Eyjafjallajokull during its entire 2010 eruption.
Update 2011may24: There is a set of excellent photos of Grímsvötn on the Boston Globe newspaper's site The Big Picture.