My concern with loaning in Bitcoin is that the supply is fixed so there's no good way I can think of for determining the interest rate. If there is a natural interaction between a finite and an infinite money based on the level of risk in the credit economy, then that gives us very important information on where to set the interest rate.
Discussion
Supply and demand, like a true open market should be.
I don't think there would be much supply though. Bitcoin holders don't like risk. They would only be willing to make collateralized loans, which are low/no risk, and only people with collateral will get loans, which creates a lot of inequity. We need to be able to take some risk or humanity will stagnate.
Also what I described is a true and open market for interest rates.