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.
If the BTCs never get moved, then they are lost. If they do someday, then it's tax evasion and jail time for you, it's very easy for the authority to set your address on watch, and sends them an alert whenever an activity is detected.