We need some kind of service where we could store our savings (making them generate profit). This is usually called BANK. So we store our bitcoins in an account, and the bank lends these bitcoins to someone who want to invest. This person pays interest to the bank, and the bank pay us that interest. Of course the bank should only lend the money for the period of time we have agreeded to save it.
This is not a fractionary reserve system because I cannot spend my bitcoins while they are lended to someone else.
Like Hawker said, it's really complicated to set up a regular loan process much less one around Bitcoin. For a Bitcoin banking industry to work, you'd need other supporting industries around it. For example, how would you handle someone who refuses to pay back their loan? You can't really send men with guns after them to put them in a cage and you can't contract with a regular private collection firm to do recovery without devaluing the transaction and losing money.
At the very least you'd need 1) a contract that specifies how much is to be repaid and when. 2) And independent arbitration firm to handle disputes 3) A collection firm the could recover your assets in case the borrower doesn't pay.
I'm not saying this couldn't work. The first two items are easy. The third could be handled by attaching the recovery process to the collection of fiat money. If the borrower didn't pay back the bitcoins, you took some of their cash if arbitration sided with you.
It's a complex thing but pretty doable if the market were to cooperate to create/extend the needed services.
Rage