Ripple Does Not Have a Concept Of Interest
There has been much discussion on whether ripple should include some kind of functionality to track interest. The current consensus is that it should not.
Ripple clients are, of course, free to have their own notion of interest accounting, but when interacting with a ripple server they make payments and that's it.
It might be possible to simulate the operation of interest, or something pretty close to it, by using multiple nodes and credit exchange rates in some creative way, but we haven't put too much thought into it. Once the protocol is established and the server is built, we expect to see clients doing all sorts of unexpected things :)