Under normal use, the Chromecast can be sent a deauth command that disconnects it from wifi. But there's a bug: when the media player is kicked off the local network it enters a config mode and becomes a wifi hotspot - waiting for machines nearby to connect with it and send it a new configuration.
Which is enough to make you feel let-down, and to make you cry and say goodbye, quite frankly.
This hack is the work of Dan Petro, a whitehat at security consultancy Bishop Fox. He's using a Pi, a couple of wifi cards and a touchscreen - along with Aircrack (open-source WEP and WPA-PSK-cracking software). It takes the device about thirty seconds to connect, take over the network and get Rickrolling; and, of course, it has to be within wifi range. You can watch a video presentation from Dan that goes into much more depth about the project on YouTube.