The Austrian vs Keynesian end of the blogosphere often battle over the existence of a natural rate of interest. The Keynesian side typically points to Piero Sraffa's argument that there are many natural rates of interest, or own-rates, and therefore an Austrian sort of monolithic natural rate of interest simply doesn't exist.
Over the last few weeks I've participated in the comments here at Jonathan Finegold Catalan's blog and here at Daniel Kuehn's blog. Here is an older comment in this vein on "Lord Keynes" blog. Bob Murphy also has a paper (pdf) on this subject and has commented on the above blogs on this subject.