It depends on what legal definition of theft you take. In the British legal system Oxford v Moss established that copying information without physically taking the physical media is not theft. However there is the principle in R v Beecham shows that once the 'value' or 'virtue' of something is gone, even if you return it, you may have committed theft since the property is no longer of use to the owner. So copying a CD, the value of the original item still has it's value. If I take your credit card out of your wallet, post the details on here and then return it, it is now essentially useless to you as a credit card. Now defining appropriation in the context of cyber crime is even more of a headache...