The Mobile Device Detector is a commonly used module for doing server-side detection of devices, and serving a different Device layer accordingly.
It's based on the Sitecore Rules Engine, which underwent significant changes in Sitecore 7.1. Once you know the new structure, it's not difficult to update the Mobile Device Detector rules to work on 7.1. But if you'd like to save some time, download the package below. Install it after the standard Mobile Device Detector package to fix up the rules.