I was envisaging the exchanges paying interest to build their reserves. For example. Bitcoinica is short of USD at the moment.

Currently, lending Bitcoins to a person you met on the internet seems insanely risky, there is no enforceable way to make the receiver repay the funds. Maybe if they signed a contract it would help, or had other assets in escrow as security for the loan.
Lending BTC to a company (eg. an exchange) might be possible for small amounts, as they put their reputation at stake.