Aetna and the American Medical Association are in a dispute centering on how much Aetna pays out-of-network doctors in some instances and the right of those doctors to bill HMO members for charges the insurer doesn't pay. Millions of dollars are at stake, and physicians say Aetna is violating a provision of its 2003 nationwide settlement with doctors. Aetna says it is only trying to protect members from high charges.