I think that scenario is pretty unlikely for large exchanges.

If someone gave me a copy of a private key, corresponding to some bitcoins, I wouldn't consider it payment, as they still have the private key as well (meaning they could still spend the bitcoins at any time).

It's more like a "shared account" at that point.

Not if the private key is stored in a secure hardware token.

Would you be willing to bet $147M that someone hadn't figured out a way around that hardware security? I wouldn't..

Can you tell the difference between a secure hardware token and a fake secure hardware token?

I suppose if you try to extract the secret key and the token destroys it, it's pretty secure. Of course, you just lost all your money. I can't think of a non-destructive test.

How does the key get into the token, and how do you know there wasn't a copy made before that?

For example Trezor hardware wallet ( http://www.bitcointrezor.com/ ) generates its private key when it is first initialized. This way, the key never exists outside of the wallet.

Not sure that solves he upthread probem. If I give you a Treznor hardware wallet containing a private key to $150million worth of bitcoins - would you trust me not to have anther copy of that key? Or would you transfer them immediately to a wallet with a private key I could never possibly have known?

Trezor supports exporting the key for backup purposes.

SIM card f.ex.