Essentially you are entering into a contract with a loan and with a thorough screening process you could have enough information to go after the person in court fairly easily (assuming they were in your country). The fact that the contract is in bitcoins doesn't matter, as a court would rule on any sort of contract, whether someone promised to pay in Euros or gold or chicken, so long as you could prove that the value of the dispute is greater than $20 (in the US, at least 7th Amendment).

I think the concept of a bank is a fairly reasonable one. Individuals deposit their BTC in a "wallet" account setup through the bank site. Loans come from individuals looking to purchase mining hardware and present a proposal of what they need, the cost, the estimated Mhashes, current mining group and number of hours per day the rig would run along with identification information, passport, driver's license, ss card, proof of income, record of any collateral, btc and otherwise. Based on this, the current and future estimated difficulty and exchange rate, an interest rate and repayment plan would be devised that allows for a generous margin for both profit and potential risk. Depositors have basically invested their money in future mining expeditions and the bank profits by assuming the risk and collecting extra overhead as a service fee.

From a risk perspective, it would be best to initially limit investors deposits to only pay interest if they were invested in a CD-type format, for a period of 3-months, 6-months, 1 year etc with those withdrawing early receiving a substantial reduction in paid interest. Once the bank has a substantial amount of guaranteed capital, faith in the bank would be increased and it could eventually offer interest on regular "checking" accounts based on average daily balance. A limit to account size could be put in place to ensure no single investor could create a bank run. Other restrictions, such as limiting the number of new accounts offered in the event of low loan demand could be established. Interest paid can be adjusted based on exchange rates and difficulty values.

I think it's a fantastic idea. The bank itself would need a fairly substantial capital backing when it first opens, in BTC and probably a "real" currency like USD. I'm very interested in this and if anyone has the programming skills to create such a thing, I'd be happy to pay you, in American greenbacks.