A related question:
https://bitcointalk.org/index.php?topic=117551.0Like in the case of "stealing", you need to be careful about the word "property".
As I understand it you cannot steal bitcoin, but you "can" commit various other crimes when you try to access bitcoins/keys which you are not authorized to.
Similar "property":
E.g. my credit card doesn't even belong to me. The plastic is property of the card issuance company, and the number is something like an access code for the VISA/Mastercard services.
Given that, bitcoins are not property, but information, which is protected by authorization keys. There are two aspects to authorization: technical and legal. E.g. I can give you the private key for my savings address, but make a contract that you are only allowed to use it in case of my death. Then if you use it without my permission, you have performed unauthorized access.