Tis a grey area mate, most people I know aint covered, what you need is to make sure your bank account or credit card has enough in it to cover all damage etc, nurburgring gmbh, will take that from you on the day, should you bend any armco, need towing off etc, they wont do it through insurance. As far as i know, not many of the trackday insurers cover the ring. But like others have said as its Public road, if another car, should hit you or be deemed at fault, then there insurance will have to pay up, in which case its treated the same as normal. (police will investigate)
Drive safe and well within your limits and all should be gravy. The reason I run the munich shed is so that should the worst happen, I aint losing lots and lots of money.
try ben lovejoys site, or Northloop.co.uk for more info