Anyone wanting to know what can happen when jet engines meet volcanic ash should google BA009 Jun 24/1982 when a B747-200 lost 4 engines and had some very tense moments over the ocean near Indonesia. There was a great Mayday episode on it - might be on youtube.
I thought I read the ash cloud in Iceland was topping out at 35,000 ft? Wonder if they could open travel at FL390 and above for the crafts that can get up there?