As many people have said, the criminal law pertaining to theft in most countries will cover Bitcoin, since any theft of tangible and/or intangible items is sanctionable. The only worry comes with those jurisdictions who have an outdated definition of theft, particularly those which adhere to the continental definition of theft whereby the thief would have to physically move the location of the object to a different location than that last set by the owner. That definition would not only mean theft of Bitcoin cannot be prosecuted against, but also any theft of intangible property.
Leaving that aside, it is immaterial as to whether Bitcoin is considered as a property or a currency - currency as a form of money is still considered as property for the purpose of determining the crime of theft. Bank notes, coins can be stolen, same as any other property.
Keep in mind that theft of Bitcoin can also be prosecuted as a computer misuse crime, unless it's an actual paper wallet which is stolen for example. That in itself is a separate argument since some might say it was the paper which was stolen and not the private key itself.
Though most of what you are saying is true, most legal systems have not determined bitcoin to
be a legal property type, like fiat, coins, collectables, and etc. It is easy to say that it is, since that would
be the obvious assumption since in normal legal systems everything can fall into some form of pre-existing law,
but the true legal issue arises where when does the new "owner" of those inputs take possession of those
coins within the blockchain system? Isn't the blockchain always the owner and possessor of the coins and
the blockchain only grants you a temporary control?
The legal question is, can bitcoin be a legal property if the value token never leaves the possession of the
blockchain system? If the Bitcoin blockchain itself does not care about who owns what nor can prove who
owns what nor will correct or reverse stolen bitcoins, would it be the correct legal theory to place bitcoins
within the normal standards for property, or should a new type/term and definition be created?
For example, maybe bitcoin is a virtual item or virtual good?
http://www.forbes.com/sites/oliverherzfeld/2012/12/04/what-is-the-legal-status-of-virtual-goods/#25b527a6be21My understanding is Bitcoin is a free to participate experiment that can never grant users/miners/devs any rights or guarantees
and as such if the system fails or disappears, users have no legal right to take any enforceable action against anyone, including
the developers, since bitcoin was not intended to be a currency/asset that governmental laws would "protect" or "regulate".
When a user/miner/dev participates within the Bitcoin system, they waive any and all rights by default. If users do not agree
with automatically waving their rights when using this experimental system, then they should not participate within the bitcoin
blockchain, but only with bitcoin within regulated exchanges.
In the context of a virtual item, can it be a legally enforceable property that should fall under normal theft laws?
I don't think so, users are granted no rights by any human or governmental authority on-chain.