One thing for sure, even though the ash is probably not good for the environment itself, having a severe cut down on aircraft flights is good for nature.
It turns out that the contrails left by aircraft bounce some of the solar radiation back into space before it reaches the ground, which stops it being converted into infra-red and being trapped in the atmosphere.
So if global aviation ceased entirely, it would help global warming along quite a bit. Of course, the emissions would be gone, but their effect is long-term...
It has nothing to do with global warming, but it's a true story.
However, good point - hadn't thought of that. Maybe Gaia is swatting planes. :D
(graphic if the full page won't load: http://s3.amazonaws.com/infobeautiful/planes_volcanos.png)