Not yet and not yet. These may as much marketing decisions as other concerns. The helmets are expensive to make, difficult to package in odd shaped boxes and manufacturers are wary of consistent, long term demand.
Giro (Advantage 2) and Bell are sold in the U.S. by the same distributor, Bell Sports in Rantoul, Illinois. As such they have no interest in competeing against themselves by having two aero helmet models (although some other companies like Louis Garneau have several) for sale simultaneously and the attendant additional costs. There is rumor of a U.S. distribution of the Bell Meteor in '09 and they can be purchased from foreign web e-tailers now.
I don’t about the Specialized helmet.