In fact, there are several such backend-as-a-service offerings in existence already, including StackMob -- a ycombinator company if I'm not mistaken. And note that any such BAAS that can be used for mobile development can just as easily be used for desktop web development. More and more desktop web apps are doing most or all of their logic locally using Javascript and then accessing their servers RESTfully for data persistence. StackMob is well-positioned to take advantage of both the surge in mobile apps and these new trends in desktop web development.