The interest would be based on the risk of the investment that the lender was taking. It would probably be higher than it is today, because defaulting on a loan is a much lower risk for lenders currently in fiat than it would be in bitcoin.

Side note: I'm announcing a bounty program later today. On fridays I'll be asking what people have done to promote SN this week, and this post will be admissible even though you made it before I formally announced the bounty program.