I suppose that he has decided that he can not do any more damage to the economy, so, his services are best suited in the private sector, where they can be more fully appreciated. I hear rumors that there is a broader conspiratorial plan where the soon to be former Treasury Secretary is going to become an employee of Jim Chanos' Kynikos hedge fund. In the plan, Chanos will devise a way to get Geithner to head up a company or country that he is short, so that he can make his (short) investment realize it's full potential. ;)
Disclaimer: Any plans of conspiracy are made up and nothing more than satire.