Web Services have started to appear on servers as interfaces between business-to-business applications. To date, mobile devices have only consumed Web Services running on stationary servers. We expand this notion so that mobile devices can both offer and consume Web Services. We discuss some interesting classes of applications that can be enabled when mobile devices can host Web Services. This can be a powerful model for facilitating automatic interaction between resource-constrained mobile devices, time starved users, and pervasive infrastructure. We also explore the issues that arise due to mobility of devices hosting Web Services, such as service discovery, device disambiguation, software footprint, and security requirements. To explore this paradigm and to gain an understanding of these issues, we have implemented a retail shopping scenario where a mobile device offers wallet services to an electronic check-out kiosk. This demonstration is now housed at the IBM Industry Solutions Lab and has been operational for several months.