The April 2005 issue of Grassroots Motorsports has an article on helmets that might prove interesting to anyone who's getting ready to purchase a new helmet. In a sidebar to the main article, there is a discussion of the use of helmets in vehicles with active airbag systems. They quoted a statement from SCCA Technical services recommending that drivers should not use a full-face helmet in a car with an active airbag due to the risk that the expanding airbag can strike the chin area with sufficient force to cause fractures of the jaw.
I prefer an open-face helmet for AX and a full-face helmet for higher speed events, but I don't have to worry about airbags in my car.