Well, DD, I looked again and still on United and Airtran, they list a tank with 40# as ok. I didn't say it was cheap, but if you wanted to take them and leave them, it might be an option for some.
From the airlines with the excess removed:
UnitedAllowance/requirements
1 empty tank (max. pressure
40 psi)
AirtranWhat scuba equipment is allowed as checked baggage?
Scuba equipment with a tank is accepted as checked baggage for an additional fee of $79 (this fee does not apply to scuba equipment without a tank). All scuba equipment must be in a suitable bag.
Scuba equipment consists of the following when applying the additional fee:
one (1) scuba tank that is empty or with less than forty (40) pounds per square inch (PSI) of air pressure,