I thought you had to remove the valves also, but see this.
From
http://www.tsa.gov/public/display?theme=44&content=09000519800b4e21
Sporting Goods Scuba tanks are not allowed on commercial airlines for safety reasons. Ship scuba tanks or consider renting a tank at your destination.
However, their OWN website also contridicts the above.
from
http://www.tsa.gov/public/interapp/editorial/editorial_1190.xml
Transporting Scuba Equipment
Some scuba gear can be transported on-board the aircraft. Please refer to the guidelines below:
A compressed gas cylinder is allowed in carry-on or checked baggage ONLY if the regulator valve is completely disconnected from the cylinder and the cylinder is no longer sealed (i.e. the cylinder has an open end). The cylinder must have an opening to allow for a visual inspection inside. TSA Security Screeners will NOT remove the seal/regulator valve from the cylinder at the checkpoint. If the cylinder is sealed (i.e. the regulator valve is still attached), the cylinder is prohibited and not permitted through the security checkpoint, regardless of the reading on the pressure gauge indicator. TSA Security Screeners must visibly ensure that the cylinder is completely empty and that there are no prohibited items inside.