Mt Gox codes are exactly that - a promise of actual BTC.  As long as Mt. Gox keeps exactly the number of BTC on account that they have promised in all of their outstanding unclaimed codes all is well with the world - peace, harmony and love.  However, if people start using the codes themselves as a longer term store of value or worse yet as a medium of exchange then Mt. Gox may be tempted into issuing more codes than they have BTC and the evil dark lord of fractional reserve banking will rise again  !

As consumers we can prevent this by using the Mt. Gox codes for only a short period of time and claiming them as soon as possible.  Also ony accept actual BTC as payment (coins ok as they contain actual BTC) - never accept promises of BTC for payment.

There is already a market for the codes.  You can buy them with credit card payment at http://www.mrcoins.org