Credit has never been an alien phenomenon and it could certainly be done with Bitcoin however as we've seen with exchanges, people who blatantly abuse this kind of system and try to pretend they have more Bitcoin than they actually have etc. will be spotted easily in this kind of market.

I remember some sort of chinese exchange getting set up at one point awhile back and it had somehow gained several hundred in BTC trade volume in just a few days, of course, everyone knew that was a load of crap so they avoided it, you can expect that kind of thing to happen with credit cards.

Fractional reserve lending is very unconvincing when what people are lending out has a fixed supply, that's why all the bankers use paper now and abandoned precious metals.