Bitcoin's status as a currency may not even be relevant. Your car isn't currency, it's property. If I steal it, I will be charged with theft relative to the value of the car.
I can't see how Bitcoins would be treated any differently. They're property, and they can be readily traded at an established value, so I don't see why someone couldn't be charged for stealing them.
They did not "steal" them, they merely made a copy. The original owner still has their sequence of numbers. Snarky perhaps, but every possible argument one can make about why torrenting a video is not theft applies equally to the "theft" of bitcoins.
I'd love to see that tried in court. I have a feeling that argument won't work very well. I have some limited experience in this area, as I worked for a bankruptcy trustee for a number of years. Debtors tried all manner of convoluted arguments to escape the spirit of the law. Judges have very little patience for that sort of thing.
The judge will make a limited effort to understand Bitcoins, but the core question will remain, "what harm was done and who was it done to?" The "owner" of the Bitcoins was, unquestionably, deprived of value.
So see it however you wish: Bitcoins as tangible property that was taken, or Bitcoins as tangible property that was irreparably damaged. Either way, the judge will ask the same question. Who was harmed and how?
Escaping the conclusion that the attacker's actions are responsible for the loss of value is impossible. You can bet that the State will consider these arguments very carefully if they catch the perp and bring charges against them. From there, it's up to the defense to find a jury so dumb that they'll buy the "Bitcoins aren't property" argument.
Regarding the arguments relative to video torrents, I believe this to be an error in logic. If I copy a video from a friend, that friend can continue to derive the original value from the video. That is, the friend can continue to watch the video and enjoy it. The act is different in that the original holder of the Bitcoins can no longer use them once they have been "copied" (to use your terms).
In this case the original numbers are totally worthless since they were transfered to someone else. The original owner has been completly deprived of the property.
I can't see how Bitcoins would be treated any differently. They're property, and they can be readily traded at an established value, so I don't see why someone couldn't be charged for stealing them.