According to WordNet 2.0:
theft n : the act of taking something from someone unlawfully; "the thieving is awful at Kennedy International"
So, you take something from someone without their concent and therefor they no longer have access to the thing that was taken away (they no longer possess it). Right?
Well, assume it is right. If I tell you a joke and you tell that joke to another person. Is that theft? Did you steal the joke from me? Apperently not because I still know the joke and I can still tell it to other people. (note: if you claim you made up the joke it's plagarism, not theft).
If we extrapolate it to computer data. If I copy a file I have two identical copies, you can not distinguish them from eachother. If I move them around I'm 100% sure you can not tell them apart. Except one of the files is on my system and the other isn't. Did I steal the file? To answer that, let's go back to the beginning. You took something (a copy of a file) from someone (me). If it was with or without my concent doesn't matter (yet). The question is, do I still have access to the thing that was taken away? Yes, I still have the "original" file. So it wasn't taken from me, I still have it. Therefor it can't be theft. Because of this "they" invented something called copyright. Copyright fills the gap between theft and creating a copy without concent of the owner.
So creating a copy of a file isn't theft but a possible violation of the copyright. Ofcourse a lot of people don't agree and claim that downloading a MP3 is theft (and copyright violtation). Ofcourse the person violating the copyright is the person that made the MP3 available. You (the downloader) would be labeled the thief (by organisations like RIAA/MPAA/Brein (Dutch "version" of MPAA)). And this thing has been an somewhat redundant debat over and over again for the past years.
And today I read something more interesting.
Jamie Kellner, [...], called skipping commercials "theft" .... Uhm... okay. Then what did I take away?. As far as I can tell those commercials take something from me: time. If I skip commercials with some piece of technology it's theft. What if I leave the room or close my eyes and ears. Is that theft too? If you said yes then you deserve to get shot in the face for the simple reason that you are stealing the right to live from others simply by existing. Ofcourse I could interpret what Kellner said more figuratively. But if you are going about mangling the meaning of words then what's the purpose of give meaning to words in the first place. You should "call the beast by it's name".