Here are some resources to track the volcanic ash cloud that closed European air space in April 2010 and caused selected flight cancellations in May 2010. If you are flying in or to Europe the best thing to do is check with your airline that your flight is still on. If you can’t get a hold of your airline then you might try the web site of the airport you are flying to or from and see if flights are taking off.
Another good source of information is twitter. So I’ve included twitter accounts where I could find them.