When someone sends coins to Mybitcoin, they are now Mybitcoin's coins, and sender receives an IOU for same amount of coins in return. It is easy to say "I have 100 coins" when that person means "I have an IOU for 100 coins". If it was blatant, then it is fraud and not theft. We should all be careful when sending coins, if they dont have a reason to honor the IOUs, or don't protect their coins well enough, we shouldn't send in the first place.
This brings up an important legal point. When you have "deposited" something with a depository institution, are you a creditor with just an IOU? Or do you still own the thing and they're just storing it for you?
There are laws in this area, and what happens varies with the type of institution. For a bank, all you have is an IOU. That's why banks are so strongly regulated. For a US stockbroker, though, stock in your account belongs to you. Even if the broker goes bankrupt, the stock still belongs to you, and it had better be there. That's why Bernie Madoff is now Federal Prisoner #61727-054 and is sitting in a prison cell right now. That's also the law on money transfer firms in Japan, which is where Mt. Gox operates.
Even where assets are aggregated, as in a grain elevator or gold vault, you can still have actual ownership. There's a whole system of warehouse receipts for that sort of thing, and the law of the Uniform Commercial Code to back it up. That's been settled law for a century.
So you have more than an IOU. You have ownership of the asset. A non-bank depository institution which takes it is guilty of theft. Bankruptcy does not relieve them of that responsibility.