I am still waiting to hear about the first person who ever loses a single bitcoin stored on a paper wallet.
But I can't trade paper wallets on Mt.Gox^^
Sure you can:
mtgox->paperwallet: use the withdraw function
paperwallet->mtgox: use the import private key function
About all you can't do is list an ask for btc on a paper wallet.
Yes we can
But once they are imported you have to deal with security. 