USAir website indicates they are accepted in BOS/DCA/LGA markets

Exception cities
US Airways accepts dogs, domestic cats and birds as checked baggage on nonstop US Airways Shuttle flights in the BOS/DCA/LGA markets only. Connections are not permitted. The customer must travel on the same flight. Customers should advise US Airways when they wish to transport a pet as checked baggage on a US Airways Shuttle flight.

So there is the possiblity that a pet could have been in the cargo hold