This paper, discusses the confluence of two major recent trends in distributed information systems engineering: the evolution from static content via personalized adaptive information provisioning to Web Services, and the emergence of mobile terminals with sufficient speed to serve as parts of information systems. The combination of both trends yields the idea of mobile Web Services. While a few pioneering mobile Web Service client systems have appeared recently, this paper explores the logical next step. Whether it is feasible to use mobile terminals such as Smart Phones also as Web Service providers ("Mobile Hosts"). We first discuss the desirability of such Mobile Hosts, then discuss the challenges for design and implementation, and finally present a prototype implementation that has been developed and evaluated in cooperation with a major mobile phone vendor.