Reports of issues include completely bricked systems thanks to the update.
With every new update comes a host of solutions to former problems, but also a variety of new problems. With the 5.1 update to Apple TV though, there appear to be more problems than solutions. Users have been reporting issues ranging from a flaky Wi-Fi connection to completely bricked systems.
The best solution at this point seems to be simply not updating, or rolling back your device to 5.0. Apple hasn't released an official comment on the issue as of yet, but with the amount of problems reported, we expect a hotfix will be released before too long.