Panthera
Don't think the Polish crash has anything to do with this, European airliners have gone down before without shutting down much of the air space.
To my mind it just shows the huge difference between how EU and the US goverments view things, especially when it comes to saftey.
American businesses for the most part are left to make decisions (within rules and laws), on their own judgement,and as much of the economy here is in priviate hands, goverment tends only to get involved when there are clear risks, such as during and after 9/11/01. USA airlines though face a much more court happy society, and if something happened you can bet there would be lawsuits that blame an airline for sending the flight when it should have known better.
Europe on the other hand is much more proactive and is heavily involved in all businesses, also is much less willing to leave safety up to corporations.
The test done so far, at least according to published media reports involve flying well below the ash clouds or into gaps.
Since on one can accurately predict wind patterns, and or how long or intense this spewing of ash is supposed go on on, I for one wouldn't want to take off from JFK because things are "fine" in France, only to have conditions change during the six or so hours it takes to cross the Atlantic, what does one do then?
Some airlines are saying they may start flying more and larger planes into the open southern European airports to at least get some passengers across the pond, and some Americans home. The UK is talking about using the Royal Navy to ferry it's citizens from various points in the EU home.
As thing stand now, even if the ash situation clears up today and or the ban is lifted, it is going to take a week or much longer at least to get this mess sorted out.