I read about this - as i myself am insured with Privilege (who i can't fault for anything other than this) on Ben Lovejoy's site
http://www.nurburgring.org.uk/insurance.html
A number of companies now exclude 'de-restricted toll roads'. Although they don't name the Nurburgring, it's hard to think what else this could mean. At the time of writing, these companies are Direct Line, Privilege, Egg, Tesco and Barclaycard. I have also been advised that two more companies are on the verge of specifically excluding the ring from insurance: Royal and Sun Alliance and Norwich Union