Apple has admitted that some of its third-generation Apple TVs have Wi-Fi issues. Owners with affected hardware are now able to exchange it for a fully functional replacement.
The affected units struggle to locate a Wi-Fi network, are sometimes unable to join a network, and experience dropped or intermittent connections, reports 9to5mac. As Apple explains:
Apple has determined that a very small number of Apple TV (3rd generation) products might experience one of these Wi-Fi related connectivity issues: Cannot locate network, Unable to join network, Dropped or intermittent connection.
Official retail stores, AppleCare employees and authorised resellers have all been informed of the problem. If you have an Apple TV with Wi-Fi issues, check the serial numbers of devices with recognised issues below -- if you're affected, go visit Apple and you should be able to get a replacement.
Update: According to MacRumors, Apple has since acknowledged that it's the fourth and fifth alphanumeric numbers, not the third and fourth. Thanks spankymk!