Mt. Gox losses for tax purposes If I put $50 into Mt. Gox in February 2013, made multiples trades from BTC to USD and back, always remaining in the Mt. Gox system, and then lost all of it in the March bankruptcy, how do I handle that for tax purposes?  Do I have to pay taxes on money I never had?

There's no record of transactions since the site has been shut down.
As the final bankruptcy has not taken place, you don't know yet what you lost.  There could be some partial recovery paid to accounts.

Eventually the bankruptcy will be finalized, you'll get access to records, and you'll get the final determination of what was lost.  You'd claim your loss in that tax year, not 2013.

Edit: I see in your question you also might be wondering about paying taxes on any gains. If you have records, unfortunately you'd have to claim those as gains in the 2013 tax year.  If you have no records, you have to estimate using your best judgment.