Guys,
I'm reposting my comment from another thread as it is relevant to this thread as well.
The pseudo-anonymous nature of bitcoin will prevent anyone from legally claiming that they lost it.
So, even genuine losses like hacks cannot be offset as losses for tax purposes. What effect this will have on bitcoin as a store of wealth is something unknown.
That statement is also true for cash. How can you prove that your lost you bag full of dollar bills?
In fact, much easier to prove stolen bitcoins than with cash. You can easily prove you own the address, all transactions are public, so you can claim with proof exactly the amount stolen. Not possible with most other financial instruments. Even if you lose control (private key) of one address, you can present history of transactions with your other (controlled) addresses as ownership proof.