I was just reading about the Eyjafjallajökull eruption yesterday and the rather bold tourist enterprise sprung up to visit it, when I see it has erupted in an explosive manner. The new path to the surface for the magma opened under the ice sheet (jokull) instead of bare ground, and water/magma interactions produce copious amounts of gas. Gas is a bad thing when it comes to magma. It makes thing go boom. Instead of a lava fountain and a tourist stop in Iceland, now we have flooding as the glacier melts, and air travel restrictions as the volcano generates large plumes up into the atmosphere from the phreatic eruption. The Icelanders are concerned about the flooding and the fluorine from the ashfall. For the geology, the composition of the magma (how much silica is in it, how much gas is in it), its interaction with the surrounding rock, and how the magma changes in composition as time increases are the interesting things. For the British sitting waiting for the air to clear, realize that ash is just magma pulverized through explosive means. Blame the gas.
Here is some good information on the location and the inflation/deflation data.