Doesn't Bitfinex offer something similar to this?
From the Bitcoin wiki:
Margin trading
Their margin trading feature is unique in the Bitcoin world. Basically, it allows you to borrow funds from lenders (see next feature) to trade bitcoins. If you make a profit, you get the profit and pay the depositor interests. If you make a loss, you reimburse the depositors the whole borrowed amount + interests. [2]
Lending
Their lending feature goes hand to hand with the margin trading feature described above.
If you don't feel like a trader and/or prefer safer investments, this feature is for you. Bitfinex allows you, using your deposit wallets, to lend bitcoins and/or dollars to traders. You can put offers with your chosen terms (which rates, for how long, and how much).
Link:
http://en.bitcoin.it/wiki/Bitfinex