I did have a bit of a problem. I'll try to stick to the facts and not go into rant mode.
Checking my bags at Santa Barbara airport, (a small regional one), I waited for the TSA agent to inspect my bag on the off chance there was any questions about the 19cf pony (valve removed). He pulls it out and looks at it then calls over his supervisor. She takes one look at it and says: No way this goes. Again, I won't bore you with my arguments as I'm sure it's pretty obvious what was said.
Anyway, she gave it back to me and said it couldn't go on the plane. I was calling a friend to try and make arrangements to get it picked up and mailed back to me when the TSA supervisor walked over, asked to see the cylinder again and then decided it was Ok. So, I got lucky. I'm glad I stuck around at the Baggage inspection area and didn't just go to the gate. I'm not sure where the tank would have ended up if I weren't there to cause a stink.
So, after all this, I did some research on the rules for scuba tanks on planes, and surprised to find, as James said, in general, they aren't permitted. I tried to find a reason for the TSA agents to allow ponies with the valve removed and couldn't. If someone has it, I'd like to get a copy so I can attach it to my tank.
And finally, a careful reading of TSA policy says that the agent has final say over what goes on the plane. Even if it is allowed, they have the lattitude to pull it. Which in the big picture, is Ok with me.
Rand