I am also concerned that Tulkas is confused. Compare these two contradictory quotes.
In the first one, the borrower returns the principal's value in USD. In the second, the borrower returns the borrowed amount about regardless of the value of USD.
You borrow 30.0 BTC today, to pay next month, suppose BTC is at USD$30.00.
Next month you will make the payment, BTC price is now USD$ 60, instead of paying 30 BTC back, you will pay 15 BTC, the amount in MtGox USD remains constant.
If you borrow 10 bitcoins today, to pay next month, you will theoretically pay aprox. USD $300.
Now suppose that in 10 days the bitcoin price jumps to USD$ 1000.00 per bitcoin, you still need to pay 10 bitcoins back, but now you will need USD$ 10,000.00 to buy 10 bitcoins!
I gave two opposite examples. The first illustrates the USD referenced loan, the other a BTC referenced loan.