Is the Ash Cloud radar reflective? And if so, at what radar frequencies? (cm, mm, etc)
Yes, it is. Depending upon the density of ash it can be visible on the cm scale, but for concentrations similar to those over northern europe you need a mm radar.
This is quite an interesting read on the subject:
http://www.ece.mtu.edu/faculty/wfp/articles/diel8.pdf
The best way to detect the ash is with lidar rather than radar, though. Only one is operational now, but there's currently a proposal to reactivate a few space based lidars for north sea/iceland overpasses. Not an easy job though and by the time the beauracracy has percolated it'll probably be too late.
Or for a/c to avoid ash concentrations themselves (like routeing around CBs for example).
Aircraft are incapable of detecting the ash (unless they're lucky enough to eyeball it). The wx radar on almost all aircraft uses too long a wavelength, so you'd need an very dense cloud.
I'm also not sure how an aircraft radar would deal with volcanic ash. It behaves differently to rain, etc, so there's a chance that much of the information could be filtered out in software before it even reached the cockpit. I don't know enough to comment further on that though.
P.S. Anyone know what the Carbon Footprint of the Volcano is? How will Iceland offset its emissions?
It's actually carbon negative right now due to the lack of aviation.
Not sulpher negative though, and I have no idea how this affects the weather...ash is a rather good absorber of sunlight.
(edit)
Surely with modern technology of satellite imagery, the airspace closure could be narrowed down significantly to just the areas closest to the plume for avoidance?
Which is what has happened. The plume is currently sitting over most of northern Europe. Worst concentrations (thick enough for me to see without the need for fancy computer software) are over the West coast of Norway, central England, southern Sweden and Holland. There's a few bits of something that looks like thick ash over Germany too, but I can't tell if it's ash or a very strange cloud.
As of an hour ago Iceland is more or less clear by the looks of things.