A plume of ash from the eruption of Iceland's Eyjafjallajokull volcano has grounded many European travelers. The volcanic ash has shut down airports as it hovered over the Atlantic Ocean and much of north and northwestern Europe. The cloud could hang around for weeks, affecting enormously busy hubs in cities like London.
The volcano spewed forth small, jagged pieces of rocks, minerals and volcanic glass the size of sand and silt. These particles could cause engine failure in aircraft, harm the plane's frame, dangerously heat up and pit its skin, scratch the windshields, and decrease visibility for pilots. Jet engines, in particular, need to "breathe" air to be heated before being expelled out the back to create forward thrust; The gases and particulate matter choke the engines by expunging the oxygen the engine needs to operate and generally gumming up the works.
On a more sciency, and slightly more depressing note, the last time this volcano erupted it lasted for more than a year (Dec 1821 - Jan 1823). As if that weren't bad enough, if the lava from the Eyjafjallajokull volcano melts the glaciers that hold down the top of neighboring Katla then that one could erupt too. The combined spewing of gases and ash would potentially cause an extended change in weather for Europe and the U.S. This is still speculation and what-ifs at this point, but its still a possibility.
Here's the story with a video: http://www.usatoday.com/money/industries/travel/2010-04-15-volcano-disrupts-europe-travel_N.htm
(image from usatoday.com/news/mapxmlfiles/ash-plume-0415.htm)