France, Belgium and Netherlands (at least) have been conducting test flights during the weekend: http://www.reuters.com/article/idUSTRE63G1U920100417

Finnish Air Force had some fighter jets in air on Thursday 15th, when the ash first hit Finland, and they've published some photos from inside the engines after some contact with ash:

http://www.flightglobal.com/articles/2010/04/16/340727/pictu...